Enquiry

[shermaine]

Hi,

Q(1)Given a amplifier circuit of a mosfet trasistor, how do we employ to determine values of c1, c2 and Cs?

Q(2) For an op-amo, what's the usage or function of resistor at + of the op-amp? Does they affect the performace of the amplifier circuit?

Q(3) what's the usage of having differential amplifier and current mirror amplifier?

 

[audioguru]

1) Where is your schematic of a Mosfet amplifier that shows C1, C2 and Cs?
2) Where is your schematic of the opamp showing the resistor?
3) Where is your schematic of ..... ???

It is easy to upload schematcs so that we will know what you are talking about.

 

[Optikon]

Hi,

Q(1)Given a amplifier circuit of a mosfet trasistor, how do we employ to determine values of c1, c2 and Cs?

Q(2) For an op-amo, what's the usage or function of resistor at + of the op-amp? Does they affect the performace of the amplifier circuit?

Q(3) what's the usage of having differential amplifier and current mirror amplifier?

Shermaine loves to make people guess at his questions. See if you can guess at these answers.

1) C1, C2 & CS are determined by bandwidth & stability considerations.

2) Usually to first order compensate for the offset error introduced by the input bias current flowing into the "-" terminal. Yes, it improves offset due to Ib but adds noise to the circuit (Johnson/Resistor noise)

3) High common mode rejection, High current source output impedance and/or for biasing other stages.

 

[audioguru]

I use opamps that have FET inputs so I never even think about adding a resistor to compensate for input bias current.

 

[Optikon]

I use opamps that have FET inputs so I never even think about adding a resistor to compensate for input bias current.

I always use FET except when

1) Gotta use bipolars because they are low noise king
2) Gotta use bipolars because that's the cheapest O/A I can get.

 

[shermaine]

Hi guys,

sorry...i forgot to upload the schematic...will upload it again. thanks!

 

[shermaine]

Hi,

I only have the 1st two question schematic.
Btw, for an op-amp, what is the concept of having virtual ground?
Please advise. Thanks.

 

[audioguru]

C1 forms a highpass filter with the source resistance in series with R1 and R2 in parallel.
C2 forms another highpass filter with the 10k load resistor.
Cs forms another highpass filter with Rs.
Each highpass filter reduces the response at the cutoff frequency by 3dB. Lower frequencies have their response reduced more.

For the opamps, R1 compensates for the bias current in R2 and R3 at the top, and compensates for the bias current in R4 and R5 at the bottom.
R1 is not needed (connect directly to the input with wire) if the opamps have FET inputs.
The inverting input of the bottom inverting opamp is a virtual ground. Look in Google.