Ok.. so I have this homework to build a multistage amplifier using 2 common emitter, 1 common collector amplifiers with 20V DC, 10mVpp ac input, and 20 Ohm load resistance. The goal is to have 60dB output. I tried to make the q point as close to 10Vdc as possible across all amplifiers but I always end up with distorted cut signals with low output(in orcad). The 20 Ohm load condition seems impossible to satisfy because it severely decreases output gain(if it doesn't burst into flames). Is there a way around all of this or any organized step for building it? Or any methods that I should be aware of?

 

Welcome to AAC!

Post a schematic and restrictions you were given.

 

The cut signals (flat tops?) tell me that you are reaching the power supplies. Basically you can not exceed what power supply provides. The flats show to you that your output sine wave is exceeding the power supply. Since your power supply is 20 volts and your load is 20 Ohm, then the best you can expect across the load is 20 volts because that is the maximum that your power supply can provide. Since you want to play it safe and see nice sine wave across the load, you can say that you want 19.8 volts across the load. Apply Ohm's Law. 19.8 volts/20 Ohm=0.99 A. So. In order to have 19.8 volts across the 20 Ohm load you must have 0.99 A flowing through the load. Can you come up with a stage that can do it?

 

Welcome to AAC!

Post a schematic and restrictions you were given.

The only restrictions that were given were: 2 common emitter +1 common collector, 20Vdc, 10mVpp input, and 20ohm load. The circuit is stable at lower input signals (10uVpp for example), but it goes nuts on 10mVpp. I am pretty sure it would burst into flames in real life also. I've applied all the basic calculations for finding voltage gains and setting q point close to 10V. But is this condition feasible at all?

 

The only restrictions that were given were: 2 common emitter +1 common collector, 20Vdc, 10mVpp input, and 20ohm load. The circuit is stable at lower input signals (10uVpp for example), but it goes nuts on 10mVpp. I am pretty sure it would burst into flames in real life also. I've applied all the basic calculations for finding voltage gains and setting q point close to 10V. But is this condition feasible at all?

The fact that 10 uV signal works and 10 mV signal does not, tells me that the gain in the circuit is too high.

 

The goal is to have 60dB output.

assuming this is voltage gain, it is very difficult to do within the confines of your exercise - it is essentially a single gain stage amplifier.

if you are talking about power gain or current gain, it is much easier.

 

assuming this is voltage gain, it is very difficult to do within the confines of your exercise - it is essentially a single gain stage amplifier.

if you are talking about power gain or current gain, it is much easier.

I had repeatedly gone through so many calculations since a single change in resistance value means reconstruction of the whole amplifier circuit. Is there any easier way to do this? I am feeling that just knowing simple calculations like Av = Rc/r'e isn't enough to settle this.

 

I've applied all the basic calculations for finding voltage gains and setting q point close to 10V. But is this condition feasible at all?

Don't see why it wouldn't be possible. You have a 20V supply and can AC couple the stages.

What gain did you design for in each stage?

Is the frequency always 10KHz?

 

Don't see why it wouldn't be possible. You have a 20V supply and can AC couple the stages.

What gain did you design for in each stage?

Is the frequency always 10KHz?

frequency can be changed to anything. And gains for each stage is stage 1 = 144, stage 2 = 9.37, stage 3 = 1. My strategy was to have as much gain on the first common emitter as possible since the second stage gain is limited because of 20 Ohm load on 3rd stage.

 

gains for each stage is stage 1 = 144, stage 2 = 9.37, stage 3 = 1.

What is the impedance of a 100uF cap at 10KHz? What is the total impedance on the emitter of the first stage? What does that make the gain for that stage?

 

What is the impedance of a 100uF cap at 10KHz? What is the total impedance on the emitter of the first stage? What does that make the gain for that stage?

does capacitor have any other affect on the signal than separating dc voltage and decreasing gain(due to impedance) if it is not big enough? Sorry I am noob at this since I just learned the whole concept of amplifiers.

 

Sorry I am noob at this since I just learned the whole concept of amplifiers.

You should have been taught about adding poles and zeros to the frequency response curve.

 

Your output waveform looks screwed up. What is the maximum step size for your transient analysis?

 

You should have been taught about adding poles and zeros to the frequency response curve.

lol.. It just proves that college education sucks here in korea. I have to go learn automatic control on my own now =/

 

It just proves that college education sucks here in korea

Show how you calculated the gain for the first stage.

 

Ok.. so I have this homework to build a multistage amplifier using 2 common emitter, 1 common collector amplifiers with 20V DC, 10mVpp ac input, and 20 Ohm load resistance. The goal is to have 60dB output. I tried to make the q point as close to 10Vdc as possible across all amplifiers but I always end up with distorted cut signals with low output(in orcad). The 20 Ohm load condition seems impossible to satisfy because it severely decreases output gain(if it doesn't burst into flames). Is there a way around all of this or any organized step for building it? Or any methods that I should be aware of?

Hello there,

Audio amplifiers typically have to drive 16 ohm, 8 ohm, and even 4 ohm loads, so 20 ohms is not completely unreasonable and is rather high compared to 4 ohms.

To design this three stage amplifier, start with the LAST stage first. Get that working into a 20 ohm load with a reasonable gain and input signal. A gain of 10 should not be too unreasonable for that stage but see what you can get and if you can meet the 60db spec with that gain (i think you can but i'll wait to say more about this).

Once you get the last stage working you know it will work into 20 ohms if you have the right drive to that last stage, so then you can design the second stage so that it drives the last stage properly which means it provides an output that matches what you use to test that last stage after you design that last stage first.

Keep in mind that emitter resistor bypass capacitors can increase gain considerably and may actually cause clipping of the signal. That would mean the output will look more squarish than like a sine wave. You also have to consider your lowest frequency of operation which i am not sure if you have a spec on or not.

Amplifiers like this have a large power loss but that's the way it goes. They have been used for years up to the point where switching type amplifiers started to take over audio applications because of their drastic efficiency advantage.