# How to design a BJT Tansistor Amplifier with voltage divider biasing

Thread Starter

#### specs365

Joined Mar 14, 2019
45
I'm not sure if this has been posted somewhere before, but I can't find anything that really addresses this type of a problem. For college we need to design an amplifier circuit using a transistor, this is not that bad. The problem is that there is no proper explanation that I can find of where to start doing this given the specifications. I will post the question here. I'm not asking someone to solve it, but rather just to show me what resistor would affect what property of the circuit, and how to go about this design. Equations would also help a lot. Here is the question:

Design a small-signal AC gain stages (CE configuration) using both a BJT. Aim for the following requirements:
- Operate from a single power supply voltage
- Be suitable for either voice (300 Hz - 3 kHz) or music (100 Hz - 20 kHz) amplification
- Provide a gain of 50, and
- Drive a load of 5 kΩ (this has to be included explicitly in your circuit).

We are going with voltage divider biasing for the circuit.

#### Wolframore

Joined Jan 21, 2019
2,610
Since this is homework post an attempt of your solution. I’m guessing that you must have gotten instructions about common emitter amplifiers since its the end of the semester. You get more out of this by trying, failing and figuring out the solution. It’s not just about having the correct answer.

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Thread Starter

#### specs365

Joined Mar 14, 2019
45
Since this is homework post an attempt of your solution. I’m guessing that you must have gotten instructions about common emitter amplifiers since its the end of the semester. You get more out of this by trying, failing and figuring out the solution. It’s not just about having the correct answer.
Here is what I have, the LTSpice circuit is also included.

Like I said, I'm not here just to get an answer to a specific question. I'm am looking a a solid strategy to approach designing of common emitter amplifiers in general. This includes small projects at home, I feel that understanding the concept is more important than just knowing the answer and that's why I am here.

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#### Wolframore

Joined Jan 21, 2019
2,610
This looks pretty good, your calculations are reasonable. Do you have to calculate for overall gain of 50? I’m unclear how your design meets the design in stages requirement.

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#### Papabravo

Joined Feb 24, 2006
21,225
Let us start with a list of requirements
1. Desired gain -- voltage, current, and/or power
2. Load resistance
3. Output resistance of the signal source
4. Available supply voltages
5. Input and output resistance of the amplifier
6. Input and desired output signal amplitudes
7. Stability of the Q point with respect to variations in hFE
8. Stability of the voltage gain with respect to variations in re'
9. Environmental factors such as temperature and humidity
10. Cost, number of components, size weight, and tolerable distortion
The following are some choices you need to consider
1. Device selection
2. Choice of a biasing scheme
3. Fully or partially bypassed emitter resistor
4. Supply voltage(s)
5. Capacitor selection

Thread Starter

#### specs365

Joined Mar 14, 2019
45
This looks pretty good, your calculations are reasonable. Do you have to calculate for overall gain of 50? I’m unclear how your design meets the design in stages requirement.
So we only need one stage with a |gain| of 50. So we need the gain to be 50 with the load resistor connected. I'm not sure what you mean by the "design in satges" requirement.

Thread Starter

#### specs365

Joined Mar 14, 2019
45
Let us start with a list of requirements
1. Desired gain -- voltage, current, and/or power
2. Load resistance
3. Output resistance of the signal source
4. Available supply voltages
5. Input and output resistance of the amplifier
6. Input and desired output signal amplitudes
7. Stability of the Q point with respect to variations in hFE
8. Stability of the voltage gain with respect to variations in re'
9. Environmental factors such as temperature and humidity
10. Cost, number of components, size weight, and tolerable distortion
The following are some choices you need to consider
1. Device selection
2. Choice of a biasing scheme
3. Fully or partially bypassed emitter resistor
4. Supply voltage(s)
5. Capacitor selection
Thanks, this is a great checklist to have to make sure everything is covered. This might seem like a trivial question, but if you don't know, you don't know I guess. How does the output resistance of the signal source and load resistance come into play? What do they affect?

#### Papabravo

Joined Feb 24, 2006
21,225
So we only need one stage with a |gain| of 50. So we need the gain to be 50 with the load resistor connected. I'm not sure what you mean by the "design in satges" requirement.
The problem with designing a single stage, with a high gain, is instability. To avoid this problem, you design two stages the get to a total gain of 50 without the need for a single high gain stage. Depending on your signal levels this might require more than one power supply.

#### Papabravo

Joined Feb 24, 2006
21,225
Thanks, this is a great checklist to have to make sure everything is covered. This might seem like a trivial question, but if you don't know, you don't know I guess. How does the output resistance of the signal source and load resistance come into play? What do they affect?
The load resistance is the numerator of the expression for voltage gain. The denominator of the expression is the emitter resistance(s). That is why we bypass all or part of the emitter resistor -- to increase the AC voltage gain by making the denominator smaller.

The output resistance of the signal source determines the ability of that source to drive your input. If your input impose too great a load on the source you will essentially be shooting yourself in the foot. Think about a source with a 100 KΩ source resistance driving a 100 Ω load. If the source is 1 VAC(rms), what is the signal level across the 100 Ω load?

#### Wolframore

Joined Jan 21, 2019
2,610
So we only need one stage with a |gain| of 50. So we need the gain to be 50 with the load resistor connected. I'm not sure what you mean by the "design in satges" requirement.
IF you need overall gain then you're not done, you need to calculate impedances

This is what I was referring to: it says "stages" ".... using both a BJT"

Thread Starter

#### specs365

Joined Mar 14, 2019
45
IF you need overall gain then you're not done, you need to calculate impedances

This is what I was referring to: it says "stages" ".... using both a BJT"

View attachment 206875
I think that might be a typo as the "a" suggests singular. This might be due to the fact that they changed a few things because of the COVID-19 Lockdown we are in.

#### Audioguru again

Joined Oct 21, 2019
6,688
Your C2 is in the wrong place with a value that is too small and your output level is reduced by the voltage divider of R4 and R5.

Thread Starter

#### specs365

Joined Mar 14, 2019
45
Your C2 is in the wrong place with a value that is too small and your output level is reduced by the voltage divider of R4 and R5.
Thank you for pointing that out, now how can I fix it?

#### Audioguru again

Joined Oct 21, 2019
6,688
Thank you for pointing that out, now how can I fix it?
With C2 in the correct place, calculate its value for it to pass 100Hz to the 5k load.

#### sparky 1

Joined Nov 3, 2018
756
I remember some time ago a guy had sort of a similar approach was concerned about the fewest parts.
later he built a qrp transmitter with IRF510 final !. He got 10 Watts out. You can see the completed design how audio and RF design ended up.
I have to give that project credit collectively for getting such a substantial signal out of simple design having enough features to be useful.
http://worldwidedirt.tripod.com/gproject.html

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#### BobaMosfet

Joined Jul 1, 2009
2,113
I'm not sure if this has been posted somewhere before, but I can't find anything that really addresses this type of a problem. For college we need to design an amplifier circuit using a transistor, this is not that bad. The problem is that there is no proper explanation that I can find of where to start doing this given the specifications. I will post the question here. I'm not asking someone to solve it, but rather just to show me what resistor would affect what property of the circuit, and how to go about this design. Equations would also help a lot. Here is the question:

Design a small-signal AC gain stages (CE configuration) using both a BJT. Aim for the following requirements:
- Operate from a single power supply voltage
- Be suitable for either voice (300 Hz - 3 kHz) or music (100 Hz - 20 kHz) amplification
- Provide a gain of 50, and
- Drive a load of 5 kΩ (this has to be included explicitly in your circuit).

We are going with voltage divider biasing for the circuit.
You use a voltage divider to bias or 'center' the input AC signal into the base of the transistor such that positive and negative voltages can be reflected by the transistor without clipping. To satisfy this sort of thing, you need understan q-point, load-lines, and BJT characteristic curves. Not difficult stuff. Your instructor should have covered these things before such an assignment.

If you need help, for your reference, look at the All About Circuits educational section for transistors:

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/bipolar-junction-transistors/

for personal education, I recommend this which will cover most all your questions:

Title: Understanding Basic Electronics, 1st Ed.
Publisher: The American Radio Relay League
ISBN: 0-87259-398-3

#### MrAl

Joined Jun 17, 2014
11,469
I'm not sure if this has been posted somewhere before, but I can't find anything that really addresses this type of a problem. For college we need to design an amplifier circuit using a transistor, this is not that bad. The problem is that there is no proper explanation that I can find of where to start doing this given the specifications. I will post the question here. I'm not asking someone to solve it, but rather just to show me what resistor would affect what property of the circuit, and how to go about this design. Equations would also help a lot. Here is the question:

Design a small-signal AC gain stages (CE configuration) using both a BJT. Aim for the following requirements:
- Operate from a single power supply voltage
- Be suitable for either voice (300 Hz - 3 kHz) or music (100 Hz - 20 kHz) amplification
- Provide a gain of 50, and
- Drive a load of 5 kΩ (this has to be included explicitly in your circuit).

We are going with voltage divider biasing for the circuit.
Hello,

But did you walk into the first class and they gave you this assignment with no lecture or book to learn from?
You should really state the way you learned how to do this so we can follow.

I can give you advice but you may not want to do it that way.
For example, the emitter cap makes the gain extremely high at 20kHz, could be much much higher than 50. The way this is often handled is either you design for that right off or you use another resistor in series wit the cap.
You might even be able to design this without needed an emitter cap, but again i dont know what you were taught previously.

A way to start is to figure out how to bias the transistor properly first. Then do an AC analysis.
The two bias resistors have to bias the output usually to one half of Vcc.

The general method for designing an analog circuit of a certain circuit topology is to find one equation for each element you have to find the value for, sometimes using approximations if you dont want to do a complete exact analysis.

So to start, in words, how would you figure out the values to bias the transistor for DC?