Reguarding many of the non-idealities in op amps and transistor-based amplifiers
ID: 2266061 • Letter: R
Question
Reguarding many of the non-idealities in op amps and transistor-based amplifiers.
Explain how non-infinity gain, Vios, and Vos are related, and why the gain we
calculate is an underestimate as long as Vios is measurable (doesn’t show up
as exactly 0 on the multimeter). Measuring Vos is a good test to see if an op amp works
– why, and how do you know you have a bad one? Explain the concept of gain
bandwidth in the contexts of op amp, BJT, and CMOS amplifiers, and how this relates to
changes in gain at higher frequencies. Also explain its effect on the differentiating and
inverting circuits and the gain / bandwidth tradeoff.
Explanation / Answer
Regarding the concept of gain,in op-amp the differential gain is given as
Vo=Ad(V1-V2) which implies
Ad=Vo/(V1-V2)
Common mode gain Ac=Vo/Vc
The gain in case of a transistor is given as,
Ad=Vo/Vs=hfe*Rc/2(Rs+hie)
Output offset voltage Voos=Vo due to Vios+Vo due to Ib
This is the dc output voltage when the input terminals are grounded.
From the gain,we find that opamp and transistors have gain proportional to the output voltage.
Gain is considered as underestimate when compared to Vios because due to some mismatch in the internal circuitry,there is some output available for V1=V2 and so the common mode gain Ac is not zero. The value is very very small while Ad is very large.
In order to check an opamp is good or bad,the differential gain Ad must be an infinite value while the value of Ac must be zero. This shows that the opamp chosen is ideal for the particular operation and V1=V2.
Related to frequency,the gain of opamp,transistors etc should be maintained in a constant value.
The more positive the values of gain margin and phase margin are,the more stable is the system.
At certain frequency,the magnitude and angle conditions may get satisfied and sometimes oscillations may occur. So for this the output of the system must be a fixed value over a period of time. If not the system is said to be unstable.
When gain margin is negative at w=wpc,the system is stable while when phase margin is greater than -180 degree the system is stable.
At higher frequencies,the output voltage may vary largely causing instability in the system.
In case of non inverting amplifier,the voltage gain is always greater than 1 and the desired value can be selected by adjusting the resistance values of Rf and R1.
In case of differential amplifiers,the gain can be adjusted and can be made equal to one or less than one.