medical college test for electronics chapter

one chapter introduction to Electronics according to UHS

1. SOFTEC ACADEMY LODHRAN PHYSICS (MCAT) Electronics BY: QAYYUM NAZAR

2. SOFTEC ACADEMY LODHRAN CLASS MDCAT SUBJECT PHYSICS CHAPTER Electronics TOPICS Half and full wave Rectification Single diode for half wave rectification of an alternating current Four diodes for full wave rectification of an alternating current Operational Amplifier and its Characteristics

3. Electronics: Electronics is the branch of science and technology that makes the use of Controlled motion of electrons through different media or vacuum. P-N Junction: If the crystal of germanium or silicon is grown in such a way that it one half is doped with trivalent impurity and other half is doped with pentavalent impurity P-N junction is formed. Depletion Region: The electron diffuse in the p-type region just after the formation of P-N junction so charge less region is formed around the junction, which contain no charge carrier. This region is known as depletion region. Potential Barrier: Due to charge on the ions a potential difference develops across the depletion region, Which stops the further diffusion of electrons and holes. This potential difference is called potential barrier. The value of potential barrier for Ge is 0.3 volts while for Si is 0.7 volts. Biasing of PN - Junction: if battery of some emf is connected across a P-N junction, then junction is said to be biased junction.

4. Biasing of the junction can be done in two ways Forward biased Junction Reversed Biased Junction Forward Biased Junction: When an external potential is applied across the junction in such a way that positive terminal of the battery is connected to the P-type and negative terminal connected to the N-type then p-n junction is said to forward biased. So the external potential difference supplies the energy to free electrons in n-type region and holes to p-type region. When the energy is sufficient to overcome the potential barrier. The current may be of few milli - amperes begins to flow across the p-n junction. Diode act as closed switch, Reverse Biased Junction: When an external potential is applied across the junction in such a way that positive terminal of the battery is connected to the n-type and negative terminal connected to the p-type then p-n junction is said to reversed biased . No current flow due to majority charge A very small current in micro ampere flow across the junction due to minority charge carrier which called reverse current of leakage current. Diode act as open switch. Diode: P-n junction is also known as semiconductor diode. It can be symbolically represented as shown. The arrow head represent the p region while vertical line represent the n-region. When current flows in direction of arrow then diode is called forward biased.

5. Rectification: The conversion of alternating current into alternating current is called as rectification. The circuit used for this purpose is called as rectifier. we use diode for this purpose Rectifiers are off two types a) Half wave rectifier Full wave rectifier Half Wave Rectifier: The circuit which converts the half of the alternating current cycle into direct current is called as half wave rectifier circuit. During Positive Half Cycle: During the positive Half cycle (0 T/2) of the input a.c signal, the diode is forward biased, so offer low resistance and a current flow through During Negative Half Cycle: During the negative half cycle (T/2 T) of a.c input signal, The diode is reverse biased. So it offer a very high resistance and current flow through R is almost zero. It has direct current which has pulse. The frequency of ripple/pulse in output is equal to f of input A.C The voltage which appears across the load resistance is known as output voltage.  

6. Full wave Rectification: The circuit used for obtaining the D.C signal for both the half cycle of input A.C is called as full wave rectification. A full wave rectifier circuit consists of four diodes in form of a bridge. That is why it is called as bridge wave rectifier circuit. During the Positive Half Cycle: During the positive half cycle (0 T/2) of A.C input the terminal A of the bridge is positive with respect to B. so diodes D1 and D3 becomes forward biased and conduct. Direction of I through R is shown During The Negative Half Cycle: During the negative half cycle (T/2 ) of A.C input. The terminal A of the bridge is negative with respect to B so the D2 and D4 becomes forward biased and conduct. The direction of current through R is shown. It is clear from the figure a and b that the current through R flows in the same direction for both the half cycle of input A.C signal. However the output is pulsating D.C. the pulse can be removed by using filter circuit Filter circuit is made from the suitable combination of capacitors and inductors  

7. Frequency of the output pulse is twice the frequency of A.C input wave. When D1 and D4 are off then no output ie the circuit is incomplete. Operational Amplifier: It is an electronic circuit used in electronic instrument Amplifier integrated on a small silicon chip and enclosed in a capsule. Pins for input output, and power supply etc are projected outside the capsule. Operational amplifier is used to perform mathematical operation electronically. Inverting Input: ( - ) A signal applied at inverting input appears after amplification at output terminal with the phase difference of 180. Non Inverting Input: (+) A signal applied at non-inverting input appears after amplification at output without any phase change.

8. Characteristics of OP-AMP : High input Resistance: It is the resistance between + and – input of the amplifier whose value is of the order of several mega ohms (2M Ω or more) so due to large value of Rin practically no current flows between the two input terminals. In ideal case Rin is infinite. Low output resistance: It is resistance between output terminal and ground. Its value is only a few ohms. It is low about 100 Ω so practically op-amplifier can deliver all its output voltage to a load of 2K Ω or more connected to amplifier, which is an advantage. Ideal op-amp has zero output resistance. Open loop Gain: It is ratio of out put voltage V0 to voltage difference between inverting and non inverting inputs (V + - V - ), when there is no external connection between the input and output. A OL = = The open loop gain of an amplifier is very high of the order of 10 5  

9. OP-Amp as Inverting Amplifier: Input voltage V1 is applied across R1 to inverting terminal (-) The output voltage V0 is therefore out of phase with input The non inverting terminal is at ground (zero potential) Open loop gain is very high of the order of 10 5. thus V may have any value between + Vcc and – Vcc . AOL = = for any value V0 V + - V - = 0 or V + = V - at ground Current in R1 = = = Current in R2 = = = Gain of non-inverting amplifier: G = = - Output signal 180 out of phase with respect to input signal Close loop gain depends upon the value of R1 and R2 is independent what is happening inside the amplifier.  

10. Operational Amplifier as non Inverting Amplifier: Consider the circuit of non inverting amplifier. The input voltage Vi is applied across the non inverting terminal + . - ve terminal is grounded. G = = 1 + Gain depends upon the two extremely connected resistances It is independent of external structure + ve sign.   What is the current in the circuit shown R =300 ohm 0A b) 10 -3 A c) 1 A d) 0.1 A -1 V is high with respect to -4 V and as the diode is reversed biased so current is zero.

11. The current supply by battery is R1 =20 ohm R2 = 10 ohm 0.1 A b) 0.2A c) 0.3 A d) 0.4 A I = = = 0.2 A   Given V in = 50mV of inverting amplifier. Two amplifiers are connected as shown. The value of V is 0 V b) 10 V c) 200 V d) 2×10 6 V G = => 20 = => Vo = 1 V G = => 10 = => V0 = 10 V   Gain of given circuit is 4 b) 5 c) 0.25 d) 3 G = 1+ = 1 + = =5  

12. Of the diode shown in the following diagrams, Which one is revere biased . In the diagram, the input is across the terminal A and C and the output is across the terminals B and D then output is a ) Zero b) Full wave rectification c) Same as input d) Half wave rectifier The current through an ideal P-N junction shown in the following circuit diagram Zero b) 10 mA c ) 1 mA d ) 30 mA as V = 2 V is greater than 1 V so reverse biased.

13. The maximum forward current rating is 4mA for diode is a) safe b) unsafe c) May or may safe d) None of the above I = = = = 0.004 Ω = 4×10 -3 A safe