[SOLVED] Trouble getting common emitter amplifier to work as a vas stage

[Simon-Pedersen]

Hello.

I have been given the task of designing a 3 stage 300 watt audio amplifier. The amplifier is suppose to contain a differential input stage, a voltage amplifying stage and a class AB output stage.

I have nailed the first and last stage, but can't seem to get the design of the VAS stage to work.

The design I am trying to achieve has the following specifications :

1. The operating point given from the differential input stage is 45 Volts, with a signal swing of 14 V
2. A Collector current of 1 mA
3. A overall VAS stage gain of 1000 V/V
4. VCC+ and VCC- 60 V
5. Delivering the output signal to a load of 100k ohms.

Given the fact that I know that there is 1 mA flowing through the collector, I choose the Resistor(RC) to be around 60 K, this ensures that when no input signal is applied to the VAS stage the output should be zero.

From there i calculated what the emitter resistance(RE) should be based on the following equation :


\[
\begin{equation}
AV =\frac{ \frac{RC*RL}{RC+RL}}{re+Re}
\end{equation}
\]

Only problem is that when i simulate the circuit using LTSPICE, i don't get a 1000V/V amplification at all, in fact i get an attenuation.
I have uploadede some pictures of my circuit and the input and output.

I Hope someone can point me in the right direction, i have been hammering at it all day,

 

[barry]

First of alI, i dont think you want that current source in the emitter. When you say the collector current 1ma, what do you mean? At zero input? Can you ac couple the input?

 

[Audioguru]

The voltage gain of a transistor is roughly the collector load impedance (yours is 37.5k) divided by the emitter load impedance. Your emitter load is a current source with a very high impedance so your transistor has no voltage gain.

 

[BigBoss]

DC Current Sources are Open Circuit for AC signals, therefore you don't get amplification.