Predicting Amplifier Output.

[lddang]

Hi I am new to op-amps design. I would like to know how can I mathematically predict the output of a design. I want to setup the design based on the suggested design but dont know how to select for the different values of the components.

 

[barry]

Are you asking for somebody to give you the transfer function of that circuit? We can do that, but I'm not sure that's really going to help you. Unless you understand what that circuit really does, you are not going to be able to make intelligent selection of component values.

Depending on the values, that circuit can be thought of as a 'leaky integrator', or an inverting amplifier with some low-pass filtering or a charge amplifier (as the title suggests).

 

[lddang]

I'm not asking for the transfer function, I want to know how to approach in a analysis level.

 

[barry]

If you don't have the transfer function, how will you analyze it?

 

[lddang]

Ok I am working out on the tf please correct my logic. Since this is an inverting I am using the equation A=-Zf/Zs where Zf is R1||C1 which is R1/(1+s*C1*R1). I am a little confused on R2 and R3. How I am treating those to resistors?

 

[barry]

Charge amplifiers are a little different, and I'm not an expert on them (maybe someone smarter than me can enter the discussion here). Forgetting about r2/r3 for a minute, the output voltage is simply: -(Qin/C1) where Qin is the input charge. R2/R3 will add a positive offset voltage on the output, R1 provides a discharge path for C1.

 

[lddang]

I'm assuming Qin (input charge) is the voltage generated from the sensor in this case?

 

[barry]

Qin is just what it says: charge, NOT voltage. The relationship to keep in mind is:

V=Q/C