Voltage-controlled amplifier

[paseka]

Hi everyone, can someone help me analyse this circuit?
It is a Voltage-controlled amplifier using an op-amp(741) and a JFET(2N3819). Please provide all info. regarding :the component terminal connections, the formula for determining the gain using the compopnent numbers in the circuit; limitations of a 741 when used as an amplifier. Thanks in advance

 

[(*steve*)]

If Q1 was shorted out, the gain of the stage would be R2/R3.

Q1 adds to the resistance of R3 by an amount determined by the gate voltage. So changing the setting on the pot changes the voltage on the gate of Q1, which alters one of the resistors in the feedback loop of the amplifier, which changes the gain.

 

[paseka]

Voltage-gain

Thanks Steve for your kind reply. For additional info, the junction field-effective transistor used herein is 2N3819. What must I do to get a voltage gain of exactly 5 if I adjust the variable resistor?

 

[(*steve*)]

The variable resistor must be adjusted until the resistance across the FET is equivalent to R2/5 - R3

Alternatively (and perhaps more pragmatically) the pot needs to be adjusted until the voltage gain is equal to 5.

Without component values, and understanding that the characteristics of FETs vary somewhat from device to device it is hard to be particularly precise.

 

[paseka]

Ok, now let's add values:
C1= capacitor between 1 and 10 microfarad
RV1= 47k ohm; R1= 47k ohm; R2= 33k ohm; R3= 2200 ohm; R4= 47k ohm; R5= 10k ohm.

How is Vgs(off) calculated?

 

[(*steve*)]

Vgs is a value you read from the specs of the device.

How they calculate it is probably of little concern. Essentially they make sure that the quoted voltage will not break down the diode formed at the gate junction.

edit: JFET, not MOSFET (so diode, not insulating barrier)

 

[(*steve*)]

With those values, the gain will be variable from 1 to approx 10.