Amplifier problem


Joined Aug 24, 2008
The design requires a relatively high frequency cut off and a very large gain. This can only be done using a cascade of several stages. With such a high gain stability is a problem. Each stage will therefore need to be decoupled with regards to the supply. Otherwise the supply line will become a feedback path and intoduce oscillations.

My advice is to use 3 or 4 stages of BJT difference amplifiers each having a gain of 10 or less. This gives them a higher cutoff frequency than when using fewer stages having higher gain. Also it is easy to stabalize this type of amplifier using simple RC filtering at each stage to decouple it from the supply.

More if you want. :)


Joined Nov 9, 2007
Looks like the culmination of a semester's work.

PRS suggestion of difference amps (long tail pairs) is a good one, but also remember that although you can't use integrated amplifiers, you can borrow techniques from them to help things along. So expect to include configurations like active loads, current sources in the tail, and cascode connections to gain the frequency response.

Keep us posted.