Amplifier Stage Configuration

[aabundle]

Hey guys,

Im doing a project for UNI and have to make a 3 stage amp for a microphone (Rout = 10k, open cct V of 1mV pk) that drives a 15Ω speaker. I have 3 amps to use, a NPN BJT, PNP BJT and a p channel JFET.

My problem is i have made a two stage amp with the two BJT's just connected together with the output amp being set up in the common emitter configuration, but i dont know where i should put the JFET, i can leave it out but i want to get a high gain and bandwidth from the amp. Would the JFET increase my gain?

Any suggestions? Ive seen on car head unit that it says MOSFET output and such but i don't know what would be the best configuration for 2 BJT's and a JFET.

 

[Hero999]

I think the idea was to use the JFET for the input so it has a really high input impedance.

Please post a schematic of what you've done so far.

 

[aabundle]

ok cool,

will scan today

 

[aabundle]

Hey here is the set up i have tried. The picture im sorry for i just did it paint because its a public holiday here today (its pretty bad but i think you will get the idea).

I did all the calcs for the circuit and have labeled them on the diagram. I aimed for a gain of 50 on the first stage and i think that made the amp results come out a bit poor.

Rin = 7.3K
Rout = 230

**broken link removed**

Even my 2 stage design is highly flawed. Any tips from any pro's out there. I can do all the calcs for single stages ok but as soon as i add more stages i start to bugger things up. Cheers

 

[aabundle]

**broken link removed**

if you cant see the picture which i can t for some reason go to

Imageshack - cctn.jpg

 

[Hero999]

Where's the speaker connected?

Sorry but that won't work.

The PNP and NPN transistors should be connected as a high current push-pull driver to power the speaker.

The FET amplifier should be on the input.

The BJTs will provide current gain and the FET voltage gain.

You really need more transistors and won't get much gain from so few transistors.

 

[aabundle]

the speaker i thought would be connected to the Vout terminal. I will but the FET at the input terminal do the calculations and give the results and see what you think

 

[Hero999]

You mean between the output and 0V?

The speaker will then have a lot of DC current flowing through it when it should be AC coupled.

 

[aabundle]

**broken link removed**

 

[aabundle]

Hey Hero999,

So ive configured the output stage to a push pull configuration. The JFET should drive the two BJT's to half voltage so i can get the best swing from the output.

Is this how you would set it up?

Could you use cascode setup for the output?

 

[Hero999]

That's more like it, except I'd replace the resister in-between the bases with a couple of diodes.

I don't see how cascode would help, it's only audio frequencies we're interedted in - <20kHz.

You need more than three transistors to amplify a 1mV signal enough to drive a speaker.

 

[aabundle]

**broken link removed**

So these diodes effectively cancel out the voltage drops on the emitter followers and preserves the input level?

 

[Hero999]

No, I meant like this.

As I said before you need more transistors, a small audio amplifier IC such as the LM386 and a JFET pre-amplifier would be much better.

 

[audioguru]

You need to amplify the 1mV pk input to 10V pk output. So you need a voltage gain of 10,000.
But the Jfet is the only part in your circuit that has voltage gain and its gain is only about 20.
You need to add a preamp that has a voltage gain of 500.