Hello,

I am new to this forum. I am having trouble coming up with a woking design for my college level Electronic Devices and Circuits course that I am taking. Here is the problem:

Design a 2-stage BJT Amplifier with a 3.5V peak-to-peak output. The input is a 70mV (RMS) signal. The load it will drive will be a 1 megaohm resistor. Minimize all noise.

I recently built a single stage stable amplifier that amplified a 100mV signal at 1KHz to about 1.30V output, but I can't seem to come up with a dual stage amplifier. Would a diff amp work for this problem?

Any help would be appreciated!

 

What is the trouble you are having?

When you build multiple stage transistor amps you usually need to couple one stage to the next with a coupling capacitor. And, the stage gains multiply.

Why don't you post your single stage amp schematic and/or what you have tried so far.

 

I pulled this circuit off of the allaboutcircuits website. I am just having trouble calculating if this exact circuit will provide the right amount of gain that I am looking for to get the correct output voltage.

 

Look at these:
http://www.allaboutcircuits.com/vol_6/chpt_5/12.html
http://www.electronics-tutorials.ws/amplifier/amp_2.html

 

I pulled this circuit off of the allaboutcircuits website. I am just having trouble calculating if this exact circuit will provide the right amount of gain that I am looking for to get the correct output voltage.

I simulated the circuit you posted.

With an input of 1000Hz 70mV sine wave, you're not going to get 1.3V output at the speaker. All the DC voltages are listed there and the wave form is on the scope. Distortion is high and the bottom half of the sine wave is gone totally.

I think you need a major rework on this circuit.

Allen

 

Look at these:
http://www.allaboutcircuits.com/vol_6/chpt_5/12.html
http://www.electronics-tutorials.ws/amplifier/amp_2.html

I don't think theses examples are considered dual stage amplifiers?? That's one of the main requirements for this project.

Allen, what direction do you think I need to take this?

 

I found this one... it looks a lot better. What do you guys think?

 

I don't think theses examples are considered dual stage amplifiers?? That's one of the main requirements for this project.

Exactly right. These are links to help You learn to design.
There are a couple of tips in post #2

Post #7 is the right idea.

Design a 2-stage BJT Amplifier with a 3.5V peak-to-peak output. The input is a 70mV (RMS) signal.

Tip: Peak to peak output is 2.828 x RMS output. Important for calculating gain. (3.5/2.828= ?? RMS)

 

The first circuit you found wrongly has an 8 ohm load. When it has a 1M ohm load then its voltage gain is much too high.
Do you know how to add negative feedback to reduce the gain?

EDIT: The upside-down second circuit also has too much gain.

 

The first circuit you found wrongly has an 8 ohm load. When it has a 1M ohm load then its voltage gain is much too high.
Do you know how to add negative feedback to reduce the gain?

EDIT: The upside-down second circuit also has too much gain.

Do you reduce the magnitude/ and or ratio of RC and RE?

 

This is what I'm getting with my first test (using about 7V DC) this is just after the 1st stage. I tried to measure after the 2nd stage and it was completely messed up, I'll have to take a look at that. But as you can see the top is cut off of my output signal.

Edit: for some reason the picture is upside down.

 

The capacitor parallel with RE increases the gain a lot. Try the circuit without the capacitor or add another resistor in series with the existing emitter circuit.

 

This is what I'm getting with my first test (using about 7V DC) this is just after the 1st stage. I tried to measure after the 2nd stage and it was completely messed up, I'll have to take a look at that. But as you can see the top is cut off of my output signal.

1) Replace the antique transistors with 2N3904 ones.
2) Bias the first transistor properly by changing the 220k resistor to 150k.
3) Change the input level to 285mV peak or 202mV RMS.
Then the output is 3.5V p-p.

But the voltage gain is too low so you must reduce the value of the emitter resistor and change the biasing again.

 

Good news guys, I got it to work! Once took out the capacitors on the emitter out and replaced them with resistors it was pretty easy from there. I just had to tweak my resistor values for RC and RE and it ended up working really well! I was able to get all the way up to 4V (p-p) output before I got distortion.

Now, there is a part 2 extra credit to this project. I will post my current schematic and details on part 2 shortly. It is a PA system.

Thank you guys so much for your help, I couldn't have done it without you.

Jason

 

Now, there is a part 2 extra credit to this project. I will post my current schematic and details on part 2 shortly.

Isn't it supposed to be in the "homework" forum?

 

This is the circuit that I got to work. Using a 20mV (rms) signal at 1kHz I was able to get up to about 4V (p-p) before I got distortion. Perfect.

Now, I need to turn this into a PA system. I need to use my current class A amplifier to drive an 8 ohm speaker (the input would be a microphone). Basically, after talking with my professor I've determined that what I have now is essentially a "PreAmp." I need to build a power amplifier off of this to drive the speaker though... What i'd like to do is use an IC to accomplish this. Does anyone have any suggestions??

Jason

 

Do you have specs on required power output, and supply voltage available for the amp ?

Here's one:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000123.pdf