what is the Ac gain of the circuit?

[nik_hdk]

Hi,
can anyone help to to find the AC gain of this circuit.the conditions are 1) the gm of BJT is large enough 2) the mirror ratio is 1. what is the possible expression for ac voltage gain?

 

[Audioguru]

Your circuit is not an opamp because the output of an opamp has emitter-follower transistors for a low output resistance. Your circuit has common-emitter output transistors.
Your circuit has three gain stages in series and has no negative feedback so its voltage gain is very high if R2 and R3 and its load are very high resistances.

 

[D.A.(Tony)Stewart]

If R2=R3 , I recall it is Av = R2/R1 is the open loop gain.

External Closed loop would lower the output impedance and reduce the gain.

 

[Audioguru]

It is a VERY old CA3080 transconductance amplifier from 1979 or earlier. It is in my RCA IC databook but I do not have a scanner to show you.
Look at the old Harris or Intersil datasheets about it. Here is one schematic of it:

 

[nik_hdk]

Thank you guys I agree that this is not a Op-amp. but can you provide some inside about the ac gain? How av=R2/R1?

 

[Dominik Przyborowski]

R1 is a resistor degenerating input pair. So if you divide it as two series connected R1/2 resistors, You get a virtual ground between them. So The transconductance of input transistor will be Gm_in=gm/(1+gmR1/2)→2/R1 for high enough gm, while overall output gain is Gm_in·K·rout, where K=1 is current mirror ratio, rout=R2||R3. So how SunnySkyguy mentioned for R2=R3, rout=R2/2 and overal gain is equal to R2/R1 ratio.

 

[D.A.(Tony)Stewart]

Thank you guys I agree that this is not a Op-amp. but can you provide some inside about the ac gain? How av=R2/R1?

Like any Class A amplifier small signal voltage gain is always Rc/Re.

Except this is used for voltage controlled current amp or OTA. In the case of the classic CA3080 OTA , Re =0, so internal rE cannot be neglected and rE changes with external bias controlled current sink. Due to current mirrors, output current is also modulated, in CA3080. Since R2, R3 is open , effective internal gm value is determined by current mirrors controlled by input. Thus you get 2 Stage variable voltage controlled current gain or OTA that spans several decades,suitable for any OTA gain or wide range(60dB) AGC control with nominal current Av set by external R2.

 

[nik_hdk]

Thanks a lot guys:smile: