BJT, MOSFET biasing and AMPLIFIER design

Load line, operating point, biasing methods for BJT and MOSFET, BJT small signal model – Analysis of CE, CB, CC amplifiers- Gain and frequency response –MOSFET small signal model– Analysis of CS, CG and Source follower – Gain and frequency response- High frequency analysis.

1. https://slideplayer.com/slide/12840513/ https://www.tutorialspoint.com/basic_electronics/basic_electronics_transistor_load_line_analysis.htm https://www.iiitd.edu.in/~mshashmi/CMOS_2015/index.html Dr. C.Helen Sulochana Professor/ECE ELECTRONIC CIRCUITS EC22301

2. Load line, operating point, biasing methods for BJT and MOSFET, BJT small signal model – Analysis of CE, CB, CC amplifiers- Gain and frequency response –MOSFET small signal model– Analysis of CS, CG and Source follower – Gain and frequency response- High frequency analysis. AMPLIFIERS CO- To explain the basics of Amplifiers and its biasing UNIT 1 37bn7zxz

3. Transistor-semiconductor device with 3 terminals , emitter, base and collector. Current conduction is due to holes and electrons Bipolar Junction Transistor(BJT) – has two PN junctiions types  symbol Base region -very thin and lightly doped Emitter region -highly doped compared with collector Since is I B is very small

4.   Transistor configurations   Common Base(CB), Common Emitter(CE)     Common Collector(CC )   Bias - applying external DC voltage to the transistor for its operation. Input(EB jun.)- forward bias(N to – ve and P to + ve ), output(CB jun )- Reverse bias current Amplification Factor of CB is α = output ct./Input ct. current Amplification Factor of CE is β ( very high ) = α / (1 - α ) current Amplification Factor of CC is   γ Transistor used to amplify(increase the signal amplitude) the signal CE configuration is widely used in amplifier circuit Common collector

5.   Transistor output characteristics It has Active region, Saturation region, Cutoff region   Active region( linear region ) Saturation region( ohmic region ) It is the initial part of the curve In this region transistor behave as a closed switch . The collector and Emitter junctions are forward biased . I C and I b are maximum Cut off region : It is the region below the curve for I B  = 0. here collector and Emitter junctions are reverse biased. very small collector current in the transistor, even when the base current is zero (I B  = 0). transistor behaves as an open switch Output characteristic- curves drawn between output current and output voltage at constant input current. In CE, between collector current I C  and V CE at constant base current I B. region lies between saturation and cutoff. in this region, transistor acts as an  Amplifier . The transistor operates in active region when the emitter junction is forward biased and collector junction is reverse biased. μ A μ A μ A μ A μ A μ A

6. I.

7. a straight line drawn on the output characteristic of a transistor, joining the  saturation point( maximum possible collector current (I C ) and the cutoff point ( maximum possible collector-emitter voltage (V CE ) for a given load resistance (R L ) Load line, operating point Load line To find the saturation point When V CE  = 0, the collector current is maximum When I C = 0, then V CE is maximum  To find the cutoff point = 0   μ A μ A μ A μ A μ A

8. AC Load Line It is the line on the output characteristics of a transistor, which gives the values of I C and V CE when signal is given DC and AC load lines intersect at the  Q-point .  The DC load line is fixed based on the DC circuit parameters, while the AC load line can vary depending on the frequency of the AC signal . The DC load line is used to find the Q-point, while the AC load line is used to determine the output voltage limits. Load line, when drawn over the output characteristic curve, makes contact at a point called as Operating point. The operating point represents the transistor's DC voltage and current when no input ac signal is applied .   Transistor is used as an amplifier. Q-point is selected at the center of the dc load line to avoid distortion operating point (quiescent point,Q point, bias point ) AC load line

9. Factors affect the operating point Factors shift the Q point are Variation of transistor parameters- β, I CO ,V BE β is temperature dependent. As β varies ,I B varies( ). Change in I C will shift the operating point in any undesired region. V BE changes with temperature at the rate of 2.5mV/ o C. I B depends on V BE and I C depends on V BE , I B Thermal Runway Collector current produces heat at the collector junction. The raises in temperature     increases the collector leakage current I CEO This in turn increases collector current and junction temperature and the whole process repeats again. This will drive the operating point into saturation region. This process is also called thermal runway. excessive heat may destroy the transistor.

10. Biasing biasing : applying external DC voltage to the transistor to fix the operating point BJT is frequently used for amplification The operating point must be in the center (Q)of the active region for proper amplification of the signal If the operating point is shifted near to the saturation or cutoff region, it is impossible to amplify the signal without distortion

11. Biasing methods for BJT Transistor biasing : applying external DC voltage to the transistor to get the desired collector current and voltage (set the Q point is in the middle of the active region ) , so that any AC input can be amplified without distortion. Operating point will not shift into undesirable region(cut off or saturation) 1.Fixed bias 2.Collect-to-base bias 3.Voltage divider bias.  Types of Transistor Biasing Need for bias bias

12. 1 . Fixed bias (or) Base bias   Resistance   R B   is connected between supply  V CC  and base terminal of the transistor.   V CC  keeps the base emitter junction forward biased and the collector base junction reverse biased . V CC is constant, so I B depends only on R B . Value of V BE =0.7(Si) or 0.3( Ge ), V CC is much larger than V BE Using KVL in base –emitter loop Using KVL in collector –emitter loop base current (and thus the operating point ) is kept constant by a fixed voltage V CC and resistor R B . V CE and I C defines the operating point simple method that uses a single resistor (R B ) to set the base current (I B ) -- - --(1)

13. The process of making the operati ng point independent of temperature changes or variations in transistor parameters is known as  Stabilization . Stability factor measures the stability of operating point rate of change of collector current I C  with respect to the collector leakage current I CO   , base-emitter voltage (V BE ), and current gain (β).  W.r.t constant V BE , β/ I CO ,β/ I CO , β and I B  is called  Stability factor s Since transistors base current, I B remains constant for given values of V CC , the transistors operating point must also remain fixed, therefore, it is called as fixed bias . Three Stability factors stability factor should have value as minimum as possible. Stability

14. Advantages It is a simple circuit, because of small number of components. Operating point can be shifted easily anywhere in the active region by merely changing the base resistor (R B ). Maximum flexibility in the design . Disadvantages  stability factor is very high Poor thermal stability .- As temperature , β changes, the operating point also changes Usage : used in circuits where transistor is used as a switch. As temp. increases, I C Increases and hence I B decreases. Since V CC, R C, R B are fixed, I B is maintained constant when I C increases. So we cannot achieve good stabilization Therefore In fixed-bias , I B  is independent of I C General equation of stability factor Differentiating (1) w.r.t I C Substituting this in (2), -- - --(2)

15. Determine the Q-point for the circuit and draw the dc load line. Find the maximum peak value of base current for linear operation. Assume 𝛽 DC = 200 Problem saturation point - how much variation in collector current can occur and still maintain linear operation of the transistor. before saturation is reached, 𝐼 C can increase an amount ideally equal to maximum peak variation of the base current

16. 2. Collector to base bias (or) voltage feedback bias   This configuration employs negative feedback to prevent thermal runaway and stabilize the operating point. same as base bias circuit except that the base resistor R B  is returned to collector , rather than to V CC  supply  . R B is connected between base and collector to provide a feedback path. Using KVL in base –emitter loop Using KVL in collector –emitter loop   …..(1)

17. Advantages: Better stabilization compared to fixed bias . R C must be large for good stabilization Disadvantages : This circuit provides negative feedback which reduces the gain of the amplifier. General equation of stability factor Differentiating (1) If the collector current I C   increases (either due to rise in temperature or due to replacement of transistor- ), V CE  decreases due to larger  voltage drop  across collector resistor R C . Base emitter junction is forward biased(V BE =0.7). Reduction in V CE   lowers the voltage across the base resistor (R B ), thus base current I B  is reduced . The reduced base current in turn reduces the original increase in collector current I C . And keep the Q point fixed Less stability factor compared to fixed bias then …..(A) equation (A) becomes

18. Problem

19. 3. Voltage divider bias (or ) self bias (or) Universal bias Most widely used method to provide stabile operating point to a transistor. R 1 and R 2 divide the supply voltage V CC and voltage across R 2 provide fixed bias voltage V B at the transistor base. Also a resistance R E is included in series with the emitter that provides the stabilization . Since the emitter-base junction is to be forward biased, the base voltage is obtained from supply V CC  through R 1  – R 2  network Thevenin's equivalent circuit of voltage divider bias is drawn as Voltage divider bias Where thevenin’s voltage V Th is thevenin resistance R Th is

20. Using KVL in base –emitter loop Using KVL in collector –emitter loop ……( 1) V Th – I B R Th –V BE – ( I B +I c )R E =0 substituting I E =I B +I C and  I C = β I B and (1+ β ) ≈ β Equation (2) is ……(2)

21. General equation of stability factor Differentiating equation (1) w.r.t.   V Th = I B R Th +V BE + ( I B +I c )R E S varies between 1 (ideal case)   and (1 + β ) . 1 for small values of R Th /R E . (1 + β) for large values of R Th /R E For proper operation both R E  and V CC  should be larger  and R Th  small.  …..(A) Substituting in equation (A)

22. I C  does not at all depend upon β. collector current l C  depends upon V BE  but V BE  is very small compared to V th   ,so collector current I C  is practically independent of V BE . collector current I C   in this biasing circuit is almost independent of transistor parameters and hence good stabilization is ensured. Emietter résistance R E   provides excellent stabilization Advantage of self bias If collector current l C ( ) increases due to change in temperature or β , I E increases and voltage drop across emitter resistance R E increases. Therefore, V BE   decreases (equation 1) Hence I B ,  and I C decreases to hold the Q point stable due to this negative feedback action of R E

23. Problem

24. small-signal model is a modeling technique, used to approximate the behavior of non linear device( transistor) using linear equations for analyzing its behavior under small(amplitude) AC signals .  Small Signal Model and AC amplifiers   h-parameter model  is suitable for  low-frequency   analysis   hybrid-pi model  is better for   low and high-frequency  analysis Small Signal means- the input AC signal peak to peak amplitude is very small around the operating point Q The swing of the signal always lies in the active region, and so the output is not distorted. In Large Signal - , the swing of the input signal is over a wide range around the operating point. The magnitude of the input signal is very large . This drive the transistor into either saturation or cut off, so that peaks of the input is clipped(cut) since amplifier is non-linear

25. to analyze the amplifier, the transistor is replaced by linear approximation model i.e. h-parameter model ( hybrid parameters such as impedance, admittance, gain etc ) Transistor is considered as a two port network Two port network consists of 2 ports and four independent variables V 1 , V 2 , I 1 and I 2 coefficients of independent variables, I 1 and V 2 , are called as h-parameters. h(hybrid ) parameter model units of h11 and h22, are Ohm and Mho respectively h12 and h21, do not have any units h-parameter model

26. h(hybrid ) parameter model of BJT CE configuration I c = h fe I b + h oe V ce V be = h ie Ib + h re V ce CC configuration CB configuration hie =V be /I b hre = V be /V ce hfe = I c /I b hoe= I c /V ce Ie = hfc Ib + hoc Vec V bc = h ic I b + h rc V ec h ic =V bc /I b h rc = V bc /V ec h fc = I e /I b h oc = I e /V ec I c = h fb I b + h ob V cb V eb = h ib I e + h rb V cb h ib =V eb /I e h rb = V eb /V cb h fb = I c /I e h ob = I c /V cb

27. Simplifid h(hybrid) parameter model Therefore, the simplified hybrid circuit is Advantages of h parameter Relation between h parameters reverse voltage gain ( h re ) is typically very small and can be neglected output admittance ( h oe ) is also usually small The term hoe may be neglected when h oe R L < 0.1

28. hybrid pi model ( Giacoletto model)   Widely used because it is use for high frequency small signals and for low frequency small signals parameters Trans-conductance or Mutual Conductance input resistance I C  -   collector current   Thermal voltage Output resistance diffusion or base–emitter resistance Relation between h parameter and hybrid pi model

29. Comparison of Biasing circuits

30. Classification of Amplifiers Biasing-operation ,Power delivered/conduction angle The circuits used to increase the voltage, current or power of an ac signal is called amplifier. Amplifiers

31. Small signal model of Common-emitter amplifier It is used as a voltage amplifier (increase the voltage level of an input signal ) The emitter base junction is forward biased  and collector base junction is reverse biased via voltage-divider bias through resistor R 1 and R 2 . The operating point is adjusted with the help of resistors R e  and R c to operate the transistor in the active region The input signal is applied between the base and emitter through the coupling capacitor C 1 , and output is taken at the collector terminal through the coupling capacitor C 2 At low frequency (DC), bypass capacitor C E behave like a open circuit. The DC voltage drop across the emitter resistance 'R E ' reverse bias the emitter junction which reduces I B , This in turn reduces I C to compensate the raise in I C due to temperature variations. emitter resistor R E provides DC bias stability . At high frequency . bypass capacitor C E behave like a short circuit, provides a low-impedance path for AC signals to bypass the emitter resistor , significantly increasing the amplifier's voltage gain.  Bypass capacitor C E coupling capacitors block DC components while allowing AC signals to pass.  

32. When the input signal voltage at the base is increased , V B E  ( V BE = V i - I E R E )will also increase . Hence I B increases. Thus collector current I C   and the voltage drop occurs across R C increases Therefore, Output voltage decreases V o = V CC - I C R C there is a 180° phase shift between the input and output small signal analysis Consider all d.c . source as zero Replace coupling capacitors and emitter bypass capacitor with short circuit. 3. . Replace the transistor by its h-parameter model.. Steps small-signal analysis- AC behavior of the circuit.  emitter resistor R E is bypassed by the capacitor C E for AC signals, so it is often ignored in the small-signal equivalent circuit.  Note Equivalent Circuit

33. 1. Input Impedance Z i : output short circuited It is the impedance between the input terminals B and E looking into the amplifier input resistance looking into the base is Z b input resistance seen by the signal source is Z b =   Z b   Z i   -------(1) R B reduces the input impedance 2. Output Impedance Z : Z o = input open - circuited Z e   very small)   output impedance considering R C and R L is Z   Z  

34. 4. Voltage Gain or Voltage Amplification 3. Current Gain or Current Amplification: current gain is the ratio of output load current L to input current. voltage gain, is the ratio of output voltage , to input voltage A I     A I =   current gain is high A V       -------(2) substituting (1) & (2)     output short circuited input open -circuited With load resistor, R C is replaced by R C R L  

35. With Emitter resistance R E is replaced by NOTE Charateristics Applications Used as a general-purpose amplifier because of high current gain and voltage gain Audio amplifiers, such as pre-amplifiers and power amplifiers. Radio frequency (RF) amplifiers in receivers and transmitters. Low-noise amplifiers in sensitive instrumentation. Oscillator circuits. or buffer- connect a high-impedance source to a low-impedance load without attenuation ) in high frequency applications. Medium input impedance Medium output impedance High current gain High voltage gain There is a phase relationship of 180 degrees in input and output

36. It is used as a voltage amplifier (increase the voltage level of an input signal ) The emitter base junction is forward biased  and collector base junction is reverse biased via voltage-divider bias through resistor R 1 and R 2 . The operating point is adjusted with the help of resistors R e  and R c to operate the transistor in the active region The input signal is applied between the emitter and base through the coupling capacitor C 2 , and output is taken at the collector terminal through the coupling capacitor C 3   emitter resistor R E provides negative feedback, which stabilizes the operating point . due to temperature variations, if I E ( I E = I C )increases, voltage drop across the emitter resistance 'R E ' increases, which reverse bias the emitter junction and reduces I B , This in turn reduces I C to compensate the raise in I C due to temperature variations . Common Base amplifier coupling capacitors block the DC components and allow the AC signals

37. During positive half of input signal (input increases), the voltage drop across emitter resistor R E increases . This reduces the base-emitter voltage V BE (V BE = V i - I E R E ) which in turn reduces collector current I C . This reduces the voltage drop across the collector resistor Rc . . Therefore output voltage increases   During negative half cycle of input , reverse process takes place. Thus V O   decreases , and the output is negative value . there is no phase shift between the input and output . small signal analysis Consider all d.c . source as zero Replace coupling capacitors and emitter bypass capacitor with short circuit. 3. . Replace the transistor by its h-parameter model.. Steps R1 and R2 are often ignored because they are bypassed by Ce .   their impedance, viewed from the emitter is very high compared to the emitter resistance (Re ). Note V o = V CC – I C R C During negative half cycle of input , the emitter terminal is negative. Thus V BE increases which in turn increases Ic and the voltage drop across R C  . Thus V O  decreases, and the output is negative value. Equivalent Circuit

38. 1. Input Impedance Z i : output short circuited It is the impedance between the input terminals E and B looking into the amplifier input resistance looking into the Emitter is Z E input resistance seen by the signal source is Z E =   Z E   Z i   -------(1) =   Z i   input resistance is very low Z = input open - circuited Z C   very small)   output impedance considering R C and R L is Z   Z   2. Output Impedance Z :

39. 3. Current Gain or Current Amplification: current gain is the ratio of output load current L to input current. A I     A I =   -------(2)   output short circuited Current Gain is always less than 1 4. Voltage Gain or Voltage Amplification voltage gain, is the ratio of output voltage , to input voltage A V       substituting (1) & (2)   input open -circuited With load resistor, R C is replaced by R C R L    

40. Charateristics high voltage gain, low current gain (less than 1), low input impedance, high output impedance.  provides a 0-degree phase shift between input and output signals Applications of CB amplifier CB amplifier is used as constant current source due to its high output impedance and unity current gain.  used as a current buffer ( pass input current to the output with minimal loss) due to its low input impedance and unity current gain used as a voltage amplifier due to its high voltage gain (but less than CE) useful for impedance matching( match a low impedance source to a high impedance load), in radio frequency (RF) circuits and microphone , because of its low input impedance and high output impedance. 

41. It is used as a current amplifier (increase the current level of an input signal ) The emitter base junction is forward biased  and collector base junction is reverse biased via voltage-divider bias through resistor R 1 and R 2 . The operating point is adjusted with the help of resistors R e   , R 1 and R 2 to operate the transistor in the active region The input signal is applied between the base and collector through the coupling capacitor C 1 , and output is taken at the emitter terminal through the coupling capacitor C 2   emitter resistor R E provides negative feedback, which stabilizes the operating point . due to temperature variations, if I C increases , voltage drop across the emitter resistance 'R E ' increases, which reverse bias the emitter junction and reduces I B , This in turn reduces I C to compensate the raise in I C due to temperature variations .   Common collector amplifier( Emitter follower) output(emitter ) signal follows the input(base) due to the feedback mechanism in emitter resistor. Therefore, CC amplifier is called as Emitter folllower V E V coupling capacitors block the DC components and allow the AC signals

42. When the input signal voltage V i   is increased , the output voltage V   (I E R E )is increased . Since base-emitter junction acts like a forward-biased diode, maintaining constant voltage drop( 0.7V for silicon), the output voltage V at the emitter follows the input V i   at the base there is no phase shift between the input and output . V i  = V BE  + I E R E I E R E   = V i   - V BE small signal analysis Consider all d.c . source as zero Replace coupling capacitors and emitter bypass capacitor with short circuit. 3. . Replace the transistor by its h-parameter model.. Steps Equivalent Circuit

43. 1. Input Impedance Z i : 2. Output Impedance Z : input resistance looking into the base is Z b , input resistance seen by the signal source is Z b =   Z b + R E (1 +h fe )   Z i   output loop (emitter circuit) significantly influences the input impedance due to  negative feedback in resister R E .   It is the impedance between the input terminals B and C looking into the amplifier input impedance of CC is higher than the CE Z e = output impedance is determined by R E and of the transistor , source resistance Rs in series with is also considered (it’s value is larger than these resistances )     -------(1)  

44. 4. Voltage Gain or Voltage Amplification 3. Current Gain or Current Amplification: current gain is the ratio of output load current L to input current. voltage gain, is the ratio of output voltage , to input voltage Z e   Z e   output impedance considering R L and R E Z   A I     A I = (1+ℎ 𝑓𝑒 ) current gain is high Voltage gain is always less than 1 A V       -------(2) substituting (1) & (2)        

45. Characteristics of CC amplifier 1.Current gain is high as CE 2.Voltage gain is less than unity 3.Input resistance high and . 4. Output resistance is low Frequency response is high like CB , used for higher frequencies Applications CC amplifier is used as constant voltage source due to its high output impedance and unity voltage gain.  used as a voltage buffer ( pass input voltage to the output with minimal loss) due to its high input impedance and unity voltage gain useful for impedance matching( match a high impedance source to a low impedance load), used as the final stage in multistage amplifiers to provide impedance matching

46. Comparision of CE, CB,CC amplifier

47. Frequency Response of Amplifier The curve drawn between voltage gain and the signal frequency of an amplifier is known as frequency response. The voltage gain varies with frequency , because the reactance of the capacitors changes with frequency. frequency response is nearly ideal over a wide range of mid-frequency. Only at low and high frequency ends, the gain deviates from ideal characteristics. The decrease in voltage gain with frequency is called roll-off . two frequencies at which voltage gain starts decreasing below 70.7 % of maximum gain are f 1 and f 2 . These are called the lower cut-off and upper cut-off frequencies,  Bandwidth of the amplifier B= f 2  – f 1   f 1   , f 2 are called as half-power frequencies -a power level is one-half the power in mid-frequency - voltage gain of the amplifier is maximum in the midband . A mid . voltage gain of the amplifier outside the midband is 

48. At high frequencies, internal capacitances (junction capacitance between base and collector) in transistors become significant. These capacitances can create feedback paths that reduce the gain. The Miller effect can further amplify this effect, as the input capacitance appears larger due to the gain of the stage gain. As frequency reaches 𝑓 𝑇 ( high-Frequency Limit ) transistor’s internal charge storage and delay limit the gain . At low frequencies, the reactance of coupling capacitor C1, C2 and bypass capacitor C E are high and hence very small part of the signal will pass from the amplifier stage to the load, which can reduce the gain . The  capacitance Cbe  offers a low input impedance at higher frequency thus reduces the effective input signal and so the gain falls. CCe provides a shunting effect at high frequencies in the output side and reduces the gain of the amplifier. bypass capacitors are used to stabilize the DC operating point of transistors. At low frequencies, these capacitors also have high reactance, affecting the overall gain. Low Frequencies (< f 1 )  High Frequencies (> f2)   . at higher frequencies. It can reduce the amplifier's bandwidth  The Miller effect is the phenomenon where a capacitor connected between the input and output of an amplifier with gain A v appears to have a larger capacitance at the input than its actual value This apparent increase in capacitance is called Miller capacitance At higher frequencies. It can reduce the amplifier's bandwidth   Miller effect voltage gain(A) of the amplifier outside the midband is  Below midband above midband

49. R i   ’       Midband gain where Upper 3dB frequency R       Among this two, lowest (input) is the dominant one. where   )     )   The upper 3dB frequency in the input and output are calculated as )   ’     Small signal equvalent circuit of Amplifier at high frequency Hybrid pi model is used for high frequency analysis

50. R i   Lower 3dB frequency       lowest of the three is the dominant one, which is considered as lower cutoff frequency. where The lower 3dB frequency is calculated by considering the input and output coupling capacitance C1 , C2 and bypass capacitance C E     where Note: For MOSFET frequency response, diagram and equations are same. Replace R C by R D and by and capacitance C E by C S  

51. Voltage gain at the cutoff frequency of the amplifier is 200. Find the maximum gain of the amplifier. Problem. 1 Solution Problem. 2 Solution

52. MOSFET (Metal-oxide semiconductor FET) Depletion Mode When no gate voltage ( V GS ) , the channel has maximum drain current (I D ) When the gate voltage is either positive (p channel) or negative (n channel), then the I D decreases . N channel P channel D MOSFET E MOSFET Source (S ) Drain (D) Gate (G) 3 terminals Emitter collector base N channel

53. when V GS = 0 , I D = 0 there is ideally no drain current until V GS reaches a certain nonzero value called the threshold voltage , V T . when V GS > V T and V DS > V GS − V T -MOSFET works in saturation region - amplifier when V GS > V T and V DS < V GS − V T -MOSFET works in ohmic region – closed switch when V GS < V T and V DS = V DD - MOSFET works in cut off region - open switch Enhancement Mode gate voltage is positive (n channel) or negative (p channel), Transfer characteristics Output characteristics

54. BIASING THE FET The biasing of a MOSFET involves establishing operating point or Q-point in desired region of operation i.e. saturation , cut-off or ohmic regions. For amplifier applications the operating point should be located in saturation region for switching applications the operating point shifts between ohmic and cut-off regions. Need of biasing 1.Fixed bias 2.Drain-to-Gate bias 3.Voltage divider bias.  Types of Transistor Biasing N-channel enhancement mode MOSFET circuit shows the source terminal at ground potential and is common to both the input and output sides of the circuit. *The coupling capacitor acts as an open circuit to d.c. but it allows the signal voltage to be coupled to the gate of the MOSFET

55. Using KVL in Gate –source loop Using KVL in Drain – souce loop 1 . Fixed bias (or) Gate bias Fixed DC bias is obtained using a battery V GG . For n channel, gate is always positive with respect to source and no current flows through resistor R G   since gate terminal is isolated from channel by oxide layer that is I G =0. drain-to-source voltage ( V DS ) is determined by the supply voltage (V DD ), the drain resistor ( R D ), and the drain current ( I D ).   setting a constant voltage at the gate terminal to establish a specific operating point .   the gate-to-source voltage (V GS ) remains constant , regardless of changes in temperature or the specific characteristics of the MOSFET. V GG = V GS + I G R G V GG = V GS Since I G =0 V DD – I D R D – V DS = 0 V DS = V DD - I D R D Q(V DS , I D ) is set

56. Simplicity:  The circuit is relatively simple to design and implement. Cost-effective :  It requires fewer components compared to other biasing methods.   Disadvantages Advantages Instability: The operating point is sensitive to changes in temperature and variations in transistor parameters mobility and threshold voltage (significant variations in drain current and voltage . Due to this parameters)

57.   no current flow into the gate terminal due to the insulating oxide layer between the gate and the channel   , I G = zero In a drain-to-gate biased circuit, the gate-to-source voltage (V GS ) is approximately equal to the drain-to-source voltage (V DS )   . Bias point(Q point) is varied by changing the value of drain terminal resistor R D . , independent of resistor R G Using KVL in gate –source loop Using KVL in Drain –source loop V DD – I D R D – V DS = - - - - - (1) Drain is connected to the gate through a feedback bias resistor R G The drain-to-gate resistor (R G ) provides negative feedback .  2. Biasing of N-MOSFET with Feedback Configuration feedback biasing of the enhancement n-channel MOSFET V GS = V DD – I D R D from (1) V DS = V DD – I D R D Assume MOSFET operated in the saturation region, Q(V DS , I D ) is set

58. Advantages: Stability: improved stability against variations in transistor parameters and temperature.  Simplicity: The circuit is relatively simple to implement, requiring only a single resistor.  Stabilization reduction in V GS  tends to decrease the drain current ( V GS controls the channel's conductivity), This negative feedback loop helps to stabilize the operating point Drain-to-gate feedback biasing is often used to ensure the MOSFET operates in the saturation region,  If the drain current (I D ) increases , the voltage drop across the drain resistor (R D ) also increases, reducing the gate-to-source voltage ( V GS ).  Disadvantages: Limited Gain: The negative feedback can reduce the overall gain of the amplifier circuit, MOSFET operates in the saturation region  when

59. 3. Voltage divider bias (or ) self bias (or) Universal bias5 It uses a voltage divider circuit (R1 and R2 ) to set a fixed gate voltage ( V G ), which in turn controls the  drain current  (I D ) and  drain-to-source voltage  (V DS ).  This method is particularly useful when only a single power supply is available.  Resistors R 1 and R 2 form a voltage divider, creating a stable voltage (V G ) at the gate of the MOSFET.  Since I G is zero, the current is flowing through R 1 and R 2 decide the gate voltage Biasing stability can be improved by voltage dividing bias gate voltage Using KVL in gate –source loop Using KVL in Drain –source loop V GS = V G – I D R S V DD – I D R D – V DS – I D R S = V G – V GS – I D R S = 0 https://www.youtube.com/watch?v=Ae8L9-WOiVE&t=576s Assume MOSFET operated in the saturation region V DS = V DD – I D R D – I D R S - - - - - (1) - - - - - (2) V GS is obtained by substituting the value of V G from (1) in (2) - - - - - (3) Q(V DS , I D ) is set V G

60. Stability : Voltage divider bias provides a more stable operating point compared to other biasing methods (like  fixed bias ) because it is less sensitive to variations in transistor parameters (K for MOSFETs). Single Supply Operation: It can be implemented with a single power supply, making it suitable for various applications.  I D for a given value of V GS can be found by putting the value of V GS from (2) and solving the quadratic equation Then properly choosing the value of drain resistor R D , we can ensure that the MOSFET is operated in the saturation region . If I D increases due to any reason, voltage drop across the source resistor R S increases. This reduces the voltage V GS , Then I D will also reduce. Due to this inherent negative feedback(due to R S ) , voltage divider bias is more stable compared with other biasing methods. Advantages

61. Constant current source bias Since Parameters and are temperature dependent, for robust operating point , the MOSFET can be biased using constant current source Current source ensure that drain current remain constant, even if their is a change in the external parameters. This is used in integrated circuits

62. Problem Calculate the DC operating conditions for the circuit Solution

63. Problem Calculate the DC operating conditions for the circuit Solution Calculate Calculate I D

64. Smalll signal equivalent circuit of MOSFET transconductance small-signal output resistance , λ is the channel-length modulation parameter output voltage Since Vgs = Vi , the small-signal voltage gain is since The body effect in a MOSFET refers to  the change in the transistor's threshold voltage ( V T ) due to a voltage difference between the source and the body (or substrate) of the transistor

65. common source amplifier common gate amplifier common drain (source follower ) amplifier Common-Source Amplifier Gate is the input terminal, Drain is the output and source is at ground Signal from the source is coupled into the gate of the transistor through the coupling capacitor C C , which provides dc isolation between the amplifier and the signal source. DC transistor biasing is established by R 1 and R 2. Output signal is 180 output of phase with input. Source resistance R Si should be much less than amplifier input resistance , for maximum power transfer MOSFET Amplifiers Circuit diagram Equivalent Circuit Since the source is at ground potential, there is no body effect. output voltage = ………(1) voltage gain input gate-to-source voltage small-signal voltage gain Combining (1)} &(2) ………(2) Output Resistance input resistance to the amplifier is , Input Resistance output resistance is

66. Common Gate Amplifier input signal is applied to the source terminal and the gate is at signal ground . The gate resistor R G prevents the buildup of static charge on the gate terminal, capacitor C G ensures that the gate is at signal ground. The coupling capacitor C C1 couples the signal to the source, coupling capacitor C C2 couples the output voltage to load resistance R L . small-signal transistor resistance r o is assumed to be infinite. output voltage gate-to-source voltage small-signal voltage gain KVL equation around the input, ………(1) ………(2) Combining (1) &(2) since the voltage gain is positive, the output and input signals are in phase Equivalent Circuit voltage gain Circuit diagram

67. current gain output current Input current equation Therefore, current gain then the current gain is essentially unity . input resistance to the amplifier is , looking back from the load resistance, output resistance is Since the input resistance is output resistance ………(3) ………(3)

68. Common Drain Amplifier(s ource follower) Gate is the input terminal, output signal is taken from the source with respect to ground Drain is connected directly to V DD Signal from the source is coupled into the gate of the transistor through the coupling capacitor C C , which provides dc isolation between the amplifier and the signal source. DC transistor biasing is established by R 1 and R 2. Circuit diagram Equivalent Circuit output voltage gate-to-source voltage KVL equation from input to output ………(1) ………(2) Voltage gain

69. small-signal voltage gain input resistance to the amplifier is , Therefore, output resistance is voltage V in is related to the source input voltage V i by ………(3) Combining (3) & (4) ………(4) V gs is directly across the current source g m V gs . Substituting (2) in (1) Input resistance output resistance Output and input signals are in same phase Comparison of MOSFET amplifiers