RC coupled transistor amplifier circuit.

[Wall-ED]

**broken link removed**

I want to know what the uses of R1 and R2 are besides setting the Q points, and alse the uses of Cin and Ce, and how to decrease Fl and increase Fh of the circuit, also what effect would increasing or decreasing C out make?

Thanks a lot for the help in advance.

 

[Grossel]

If you scetch the AC paths, I think the answer on question #1 will gives. The last question will also probably be given when scetching the AC path.

For question 2 I'm not familiar with the conceptions FI and Fh, then I can't answer.

 

[crutschow]

The only purpose of R1 and R2 is to set the Q point.

Cin blocks the bias voltage from appearing across Rs.

Ce provides an AC bypass across the Re so the AC gain is higher than the DC (bias) gain of the amp.

Cout acts like a HP filter that rolls off the output frequency below the RLCout time constant.

 

[MrAl]

**broken link removed**

I want to know what the uses of R1 and R2 are besides setting the Q points, and alse the uses of Cin and Ce, and how to decrease Fl and increase Fh of the circuit, also what effect would increasing or decreasing C out make?

Thanks a lot for the help in advance.

Hello there,

R1 and R2 are there to bias to the Q point but also to help stabilize for transistor gain variations from part to part. If you build 100 of these amps you'll have to use 100 different transistors, all with slightly different gains. The bias point wont change much when you do this with this particular bias scheme, and that's the strong point of using two resistors R1 and R2 instead of just R1.

As far as bias stabilization with temperature however, it's not as good as some of the other schemes unless the lower resistor R2 is relatively large, but then we end up with a circuit that is similar to a bias technique where there is only one resistor R1 and no R2.
The best for stabilization is probably the collector feedback with emitter feedback also.

To add to what Carl nicely pointed out already about the caps...

Increasing the value of any of the caps in this circuit helps boost the low frequency response. There's a catch however. If the caps are made very large values to pass nice bass sounds the power supply capacitors have to be able to support the voltage at the required current, so the power supply caps probably will have to be increased as well.

The input cap couples the AC audio signal to the DC biased transistor input. This cap also helps if the input is from another amplifier with a different DC output bias so the DC bias points dont have to be the same.
The output cap couples the DC output of the transistor to the AC input of the next stage, eliminating the need for matched DC bias voltage points.
Both the input cap and output cap eliminate the necessity to match the previous stages bias points, and this also reduces amplifier DC bias point drift as the previous stage bias point can not affect any of the following stages, which if it could, would wreck havoc on the final DC output bias point.
The emitter cap allows a large increase in AC gain while allowing the DC bias point to remain unchanged.


Just to recap a little:

Bias point stabilization depends on a number of factors, mostly emitter base voltage change with temperature and DC beta change with temperature, and also bias point change with DC supply voltage sometimes. Various schemes are devised to help eliminate or reduce the negative effects of all these so the amplifier works properly even if it gets a little hot or cold.