UNIT3- OTAs & Applications, analog electronics.pptx

OTAs , their applications

1. UNIT-3 OTA & Applications Subject- Analog Electronics-II

2. Operational Amplifier

3. Different Stages in Op-Amp STAGE 1: single ended diff amp SATGE 2: gain stage STAGE 3: buffer stage STAGE 4: class AB amplifier

4. Basic CMOS Op-Amp Design

5. Basic Structure & Functioning of Operational Trans-conductance Amplifier (OTA)

6. Gain is from M 41, to M 4, size changes k times, M 5, M 31 and M 3 are the same size M 3 and M 5 1 are also of same size, but M 5 1 Or M 3 to M 5 or M 3 one M 3 M 5 equal, but this is again k times larger size, this is also larger time, but larger of M 41, k times that k >1, the minimum value of k will be one N channel devices are similar P channel devices are similar. So, that beta N 1 is equal to beta N 2, beta P 3 1 is equal to beta P 41, assume W bias are same new Cox is same. So, these are identical. we can see from last stage that the current is going coming down and another current is going up. Current at output will be the sum of the 2 currents since these are 180 out of phase. and resulting current will be very high Larger the output current, lager is Gm, while making 1: k we have actually boosted the current by making it k times since these currents are 180 out of phase if one changes one direction the other will change the other directions. Subtraction of these currents means actually their magnitudes are added We can get higher currents for given v in1 minus v in2 or v in2 minus v in1, by pushing up and opposite signs as opposites because one will increase the other will correspondingly decrease

7. OTA Applications Programmable Resistor & OTA as Filters (High Pass, Low Pass, Band Pass, Band Reject, General Purpose Bi-Quad OTA Filter) OTA as Integrators/Differentiators (Single ended and Differential Integrators) OTA as Voltage Amplifiers OTA-C Filters (First Order and second order OTA-C Filters) OTA-C Oscillators

8. Difference between Active & Passive Filters Active Filter composed of active components like op-amp, transistor etc. Cost High, More Circuit complexity, Low Weight, External power supply required Passive Filter Composed of passive components like resistor, inductor and capacitor etc., Comparatively low cost and less complex than active filters Comparatively bulkier due to presence of inductors. External power supply not required

9. Programmable Register Rs Essentially what we are doing we are replacing R by 1 upon gm. And we know gm can be controlled by bias current. Also, because of feedback system in OTA we can make much more accurate gm values compared to passive Rs and these are much more controllable by bias currents. So, we can have large value of variation on gms , because these are tunable . We can actually tune gm values by feedbacks. So, this property of OTA is very crucial in case of active filter design.

10. How to make low pass filter tunable? If we want this omega 0 to be larger what should we do? See of course, will be normally decided by availability of Cs. So, we must boost gms . One should change the bias currents so that the low pass filter with variable cut-off can be designed Bias current is now possible from v control. So, we will change the v control bias and we have different cut-off for the low pass filter. So, it is a tunable gm-c filter, that is the advantage of OTA that OTA allows you to include such tunable quality in design of filter

11. So, now we have got 2 circuits- high pass filter with the capacitance at input and low pass filter with the capacitance at the output. f we properly make the combinations of HP and LP filters, we can make a band pass to band reject, depending on the cut-off of which, we let the frequencies pass say if low pass filter cut-off is higher than cut-off of a high pass, it will become a band pass, but if we did the other way it may reject that band between the low pass and the high pass. So, it is only a matter of adjusting cut-off for low pass and cut-off for high pass to make diff filters

12. Under certain conditions only OTAs behave like a good voltage control voltage sources. So, the condition under which OTAs behave like a good voltage control voltage sources is Gm R should be very, very high compare to one ; which many times you can always attain in any OTA The purpose of OTA is to attain higher gm. So, if you do not use very small rs then gmr will be very large compare to 1 and then equations will be same as an op-amp requirements

13. Why do we need OTA based integrators and differentiators ? The operational trans-conductance amplifier (OTA) provides highly linear electronic tunability of its transfer gain (gin), requires just a few or even no resistors for its internal circuitry, and has more reliable high-frequency performance than that of the operational amplifier. This justifies the growing interest in designing OTA-based circuits.

14. Single Ended Current Mode OTA Integrator

15. Lossy Gm-C Integrator

16. Converting single ended integrator into fully differential one

17. Mathematical analysis of differential integrator

18. Single Ended and Fully Differential OTA Based Amplifiers

19. OTA for Summing Voltages

20. First OTA-C Filters (a) Single Ended (b) Differential Implimentation

21. Second order band pass and low pass realization of OTA-C filters

22. Differential OTA-C Realization of Band Pass and Low Pass Filters

23. 3OTA2C I Oscillator

24. 3OTA2C II Oscillator

25. 4OTA2C I Oscillator

26. 4OTA2C II Oscillator

27. 5OTA2C I Oscillator

28. 5OTA2C II Oscillator

29. THANK YOU