2 interview questions - help

[sharas]

Hi
Can you please help me with the following confgurations?
For the amplifiers below, what is its -3dB bandwidth? How about its stability?
In the first figure the 2 resistors are of equal value.
Please explain how to approach these kind of questions.

Thanks,

Sharas

 

[leomecma]

The first is an inversor amplifier that has the transfer function is: Vo/Vi = -R/R. In this case it works like a inversor buffer.
The second is just a buffer ...

Both has a stability because of negative feedback, but it's stability is determined by internal feedback compesation capacitor, in this case (G= 1) the feedback is very nearly of -180º margin fase that can be non-stable if you introduce capacitive loads....

The cut frequency is determinated by bandwith frequency of amplifier

remember: const = W*GAIN

if GAIN = 1, then -3dB is just W of amplifier.


leomecma

 

[sharas]

The first is an inversor amplifier that has the transfer function is: Vo/Vi = -R/R. In this case it works like a inversor buffer.
The second is just a buffer ...

Both has a stability because of negative feedback, but it's stability is determined by internal feedback compesation capacitor, in this case (G= 1) the feedback is very nearly of -180º margin fase that can be non-stable if you introduce capacitive loads....

The cut frequency is determinated by bandwith frequency of amplifier

remember: const = W*GAIN

if GAIN = 1, then -3dB is just W of amplifier.


leomecma

Thanks for your reply.

Why does the buffer have a -180 phase margin - the feedback doesn't change the phase of the sampled signal when it is mixed back to the output?

Is it the same for the first amplifier too (regarding the phase margin)?

 

[leomecma]

Change, but when you are driving resistive loads you can see the system like a first order system, then only in high frequencies you can see this effect, internal feedback frequency capacitor compesation help reduced this effect in small frequencies. But when you are driving capacitive loads you introduce some degrees on margin fase, then you eliminated the internal compesation and it can be non-stable. Look for more details on THE ART OF ELECTRONICS on pag 242 until 248. In forum has the book for download. Look this link too, it's an Analog devices simulator to see the effect of driving capactive loads and modes of avoid the effect of non-stability. It's is true for the first amplifier too, but I think the effect isn't exactly the same, because of inverting... simule it on the link below:

**broken link removed**

Book link:
h**p://

leomecma

 

[steve_guo]

W1=Rs/(Rs+Rf)
W2=1

 

[analoger]

The -3dB bandwidth of the first figure is fu/2, the second one is fu. fu is the unity gain frequency of the amplifier.[/u]