Doubt regarding cascading.

[amit.31]

Hi all

I have a doubt regarding cascading of two stage opamp. If we have designed a High gain(> 80dB) two stage opamp, then what is the need of cascading it further.

Thanks in advance

 

[bhav]

Two-stage amplifiers are used for very high gain(much better than folded structures)
First stage – Differential amplifier
Second stage – High-gain amplifier(CS)

but i could not get what exactly u want to implement

 

[amit.31]

Hello Bhav Sir
Thanks for replying.

Actualy, I further want to increase the gain more than 80dB. So i want to cascade the 2stage Opamp with itself. I have already designed the single ended two stage Opamp (gain>80dB) whose circuit diagram file is attached at bottom. But i am thinking how should i cascade the Opamp with a single ended output to itself as the inputs are two and output is one.

I intuit that cascading will increase the gain but will reduce unity gain bandwidth to a large extent.

.

 

[jiripolivka]

You can handle your individual amplifier stages like black boxes. With opamps, the problem can be the DC component of your signal. If you need to amplify the DC component, you must make sure that after each stage it stays close to zero volts. Temperature drifts can affect it quite a lot. Then the best is to use chopper-type opamps which are stabilized even with a high gain.
If you only need the AC signal component to be amplified, use capacitive coupling between the stages. For high gain like your 80 dB, make sure you use a lowest-noise opemp as the input stage, otherwise the noise will saturate latter stages.
Designing for a high gain looks easy but the real devices are often difficult to behave as desired. Try to gradually make and connect the stages till you have a good and stable amplifier.