How To Add A Buffer Stage in a Mini Audio Amplifier

Thread Starter

makihiro

Joined May 6, 2022
3
Hello! I'm a college student in electronics engineering and I'm struggling in our project which is a Mini Audio Amplifier.

I would like to ask for help on how to add a buffer stage for my mini audio amplifier project. The instruction for this project is to create a 2 stage amplifier that will produce a voltage gain of at least 50. I have made a two-stage amplifier without a buffer and the output was very noisy. I figured that a buffer is important for an audio amplifier.

The amplifier that I previously made has a Voltage Divider Bias Configuration as the first stage and an Emitter follower as the second stage. The output was not very good. It might have been because my transistor was overheated by the soldering iron but I think it might be because the circuit is not very good as well.

Also, this amplifier will be connected to a 4 ohm and 5W speaker.

The photo attached here was my previous amplifier.

I would appreciate suggestions and ideas for this. Thank you very much!
 

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MrChips

Joined Oct 2, 2009
26,088
What is your meaning and expectation of a "buffer"?
Why in your circuit shown below you used two different transistors?
What is your expected voltage gain of the 1st stage and the 2nd stage?
Have you given any thoughts to the input and output impendances of each stage?
You omitted the value of the interstage coupling capacitor.
You show Vi but no Vo. Are we to guess where is Vo?
It is common practice to join all COMMON nodes with one line and attach a single COMMON symbol to it (or SIGNAL GND).
In every circuit diagram it is important to show component designators such as R1, C1, etc. as well as their values.
By doing so we can discuss circuit topology, function and behaviour by identifying each component explicitly.
It is much easier to identify R7 instead of saying the 2.7kΩ resistor connected between the emitter of Q2 and GND.

1651893594757.png
 

MrSalts

Joined Apr 2, 2020
1,762
Hello! I'm a college student in electronics engineering and I'm struggling in our project which is a Mini Audio Amplifier.

I would like to ask for help on how to add a buffer stage for my mini audio amplifier project. The instruction for this project is to create a 2 stage amplifier that will produce a voltage gain of at least 50. I have made a two-stage amplifier without a buffer and the output was very noisy. I figured that a buffer is important for an audio amplifier.

The amplifier that I previously made has a Voltage Divider Bias Configuration as the first stage and an Emitter follower as the second stage. The output was not very good. It might have been because my transistor was overheated by the soldering iron but I think it might be because the circuit is not very good as well.

Also, this amplifier will be connected to a 4 ohm and 5W speaker.

The photo attached here was my previous amplifier.

I would appreciate suggestions and ideas for this. Thank you very much!
Class A amplifiers are very inefficient. You have two of those in series. Also, you shouldn't expect more than a 15v peak-to-peak voltage swing from a 20v supply (likely less than 15v p-p). That said, make sure your input is less than 15v/50 = 0.3v p-p.
Are there any constraints besides gain of 50 and 4ohm output? Can you use op amps or are you stuck with discrete components?
 

Thread Starter

makihiro

Joined May 6, 2022
3
What is your meaning and expectation of a "buffer"?
Why in your circuit shown below you used two different transistors?
What is your expected voltage gain of the 1st stage and the 2nd stage?
Have you given any thoughts to the input and output impendances of each stage?
You omitted the value of the interstage coupling capacitor.
You show Vi but no Vo. Are we to guess where is Vo?
It is common practice to join all COMMON nodes with one line and attach a single COMMON symbol to it (or SIGNAL GND).
In every circuit diagram it is important to show component designators such as R1, C1, etc. as well as their values.
By doing so we can discuss circuit topology, function and behaviour by identifying each component explicitly.
It is much easier to identify R7 instead of saying the 2.7kΩ resistor connected between the emitter of Q2 and GND.

View attachment 266602
Hi, sir! Thank you for your reply. Sorry for the drawing, there should be a Vo at the collector side of the second stage.

1. I wanted to put a buffer which is a voltage follower configuration having a unity gain. I was hoping that my gain will not be very high.
2. I have not given a thought to the input and output impedances as well.
3. My teacher said that we must use different transistors. We were also given the freedom to use a FET as well but I didn't have any experience in making it.

Actually...I want to make another amplifier circuit because the circuit that I made (the one in the photo) didn't work well when I checked it with an oscilloscope. I'd like to ask for your suggestions on how I can make a two-stage audio amplifier.

Thank you so much again!
 

Thread Starter

makihiro

Joined May 6, 2022
3
Class A amplifiers are very inefficient. You have two of those in series. Also, you shouldn't expect more than a 15v peak-to-peak voltage swing from a 20v supply (likely less than 15v p-p). That said, make sure your input is less than 15v/50 = 0.3v p-p.
Are there any constraints besides gain of 50 and 4ohm output? Can you use op amps or are you stuck with discrete components?
Hello, sir! Thank you for your reply! I appreciate it. We cannot use Op amps and we are tasked to make a two-stage amplifier with discrete components only.

We can use FET but I was intimidated by using it because I don't have an idea on how to solve the resistor values for the FET...

What should I do to replace the Class A amplifiers, sir? I will change my circuit so that I can make a better one. I'm glad to receive recommendations.

Thank you so much again!
 

MrSalts

Joined Apr 2, 2020
1,762
You should read a bit about other examples of amplifiers made of discrete components.

here is a pretty good site. This is just one page for a practical class A amplifier (figure 3). There are several other examples on this site. In general, you need to amplify the voltage to the level you want and then amplify the current to handle the 4ohm load of the speaker.
https://sound-au.com/jll_hood.htm
 

Audioguru again

Joined Oct 21, 2019
4,675
You said your amplifier was "noisy". Please describe the noise. Was it a low frequency hum, a high frequency hiss or distortion?

I was taught to never bias the base of a transistor like that because a transistor with low hFE will be cutoff and a transistor with a high hFE will be saturated. My simulation of your output transistor shows that both transistors are producing severe distortion because they are biased wrong and each has a voltage gain of 360 times, then the total gain is 360 x 360= 129600 times!

I added an input bias resistor and reduced the base bias resistor values so that the bias is not affected much by the transistor range of hFE.
 

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MrAl

Joined Jun 17, 2014
8,984
What is your meaning and expectation of a "buffer"?
Why in your circuit shown below you used two different transistors?
What is your expected voltage gain of the 1st stage and the 2nd stage?
Have you given any thoughts to the input and output impendances of each stage?
You omitted the value of the interstage coupling capacitor.
You show Vi but no Vo. Are we to guess where is Vo?
It is common practice to join all COMMON nodes with one line and attach a single COMMON symbol to it (or SIGNAL GND).
In every circuit diagram it is important to show component designators such as R1, C1, etc. as well as their values.
By doing so we can discuss circuit topology, function and behaviour by identifying each component explicitly.
It is much easier to identify R7 instead of saying the 2.7kΩ resistor connected between the emitter of Q2 and GND.

View attachment 266602
You mean that last stage as the buffer or do you want to add a third transistor as a buffer?

Speakers are usually capacitor coupled in single supply amplifiers. You might get 100mw to maybe 500mw out of it depending on what transistor(s) you want to use.
There is also a push pull output amplifier but there's more to it than just a single transistor output not sure if you want to get more complicated and with two transistors for the output the biasing is more critical.
 

Audioguru again

Joined Oct 21, 2019
4,675
Your second transistor has an output resistor that is 8200 ohms. Then if it drives a speaker that is 8200 ohms the output level is cut in half. Most audio amplifiers have an extremely low output impedance so that the output level does not change with or without a speaker and the low driving impedance damps the resonances of the speaker.
Here is what happens to the 1.8V peak output level shown in post #9 when it drives a 4 ohms speaker:
 

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