# 3 Stage Voltage Amplifier

#### jnick

Joined Dec 3, 2009
1
We were tasked to create a 3 stage amplifier to connect a microphone to an 8ohm speaker, with the first stage being a FET, the second a BJT, and the third an op-amp, whose only purpose is to be a voltage follower to drive the speaker, rather then having the BJT driving the speaker. In input from the microphone is 20mV peak-peak, and we want to amplify to about 5V Peak-Peak.

The following is what I have so far. What I don't understand is why the voltage drops through the voltage follower, nor why a negative DC bias exists at prob 3, between the capacitor and the op-amp. Any suggestions/thoughts? Thanks!
http://img146.imageshack.us/img146/3897/amplifiercircuit.jpg

#### JDT

Joined Feb 12, 2009
658
The first major problem I can see is that there is no bias current to pin 3 of the op-amp. This needs a resistor to 0V to supply this current. The input current to this pin is small but is essential. Look at the 741 data sheet to find out what is is. As a rough guide, use 10k to 100k here. This will solve your problems.

The only other comment I can make, without doing any maths, is: have you got enough gain? Resistors R1 and RC could be higher values.

#### Audioguru

Joined Dec 20, 2007
11,251
The IRF510 is a power Mosfet, not a preamp. It usually uses 10V at its gate to turn it on. It might not turn on enough in this circuit with such a low gate voltage. Then it will be an extremely distorted rectifier.
Yes, the input of the opamp needs a resistor to 0V.

#### aj_silverthunder

Joined Jun 6, 2009
116
guys my doubt is" if particular voltage is not suppled to the gate of a MOSfet transistor,wont it turn on or amplify signals!?"

#### beenthere

Joined Apr 20, 2004
15,808
Maybe the IRF510 should be an MPF102. The gate will need proper biasing.

#### Audioguru

Joined Dec 20, 2007
11,251
If you do not supply enough voltage to the gate of a Mosfet then it does nothing and will not amplify.

The output current of an opamp is not enough to drive an 8 ohm speaker. Your speaker is wrong at 1000 ohms which is not available.