BJT differential amplifier

[Nerdie]

Im trying to design a bjt amplifier with a gain of 10. However, there are some problems that I cannot answer myself.

First of all, as you can see from my design, there are 2 big resistances at each base and I dont know their functionality exactly. I guess they are there for draining current from ground in roder to turn on the BJT, but why should they have to be so big resistances?

Secondly, there has to be a current source for a huge output resistance which helps the circuit to work properly I guess. However, what should be the amount of the current? Does it affect anything rather than voltages at nodes in the circuit?

Lastly, I have to design the current source with a bjt. However, I dont know how to dc bias the bjt, because of current bias which gives negative voltage levels at emitter side.

If you have an answer to any of the problems above, I would be so glad.

 

[keith1200rs]

The current source should be chosen so the collectors of the transistors settle at a sensible voltage. In particular you don't want so much current that they end up at 0V. So, with 5k and 12V supply you might choose a 6V or 3V drop across the collector loads. So, 0.6mA for 3V drop. That is per collector - the current source would therefore be twice that. If you use too little current the collect swing will be very limited.

The 1M resistors in the base are needed to provide DC bias for the transistors. With 5k load the 1M looks rather high. You generally want a high input impedance and low output impedance. As your output impedance is 5k and therefore considered "low" in your circuit, you would normally be using something larger for the bases.

The ciruit you have drawn implies a dual supply. In other words the emitters need to be below 0V so the current source will also nee to be below 0V. If you want to use a single supply you could bias the bases at say 3V using two resistors instead of one. Then emitters will be above ground and a single supply can be used.

Keith.

 

[Nerdie]

Actually the ac supply is not a supply but acts like an incoming message

 

[keith1200rs]

Actually the ac supply is not a supply but acts like an incoming message

I am not quite sure what you mean by that, but I don't think it alters what I have already said.

Keith.

 

[Nerdie]

I mean they are the signals coming from transducers( sound signals), but this still makes them the supply
And what about the current source bjt model? How should I dc bias it?

 

[keith1200rs]

It is upside down, but I would suggest something like this circuit using Q1 should do:

**broken link removed**

You need to do the equivalent with an NPN referenced to 0V.

Keith.