# I need your help on CE amplifier

#### PaulEngineer

Joined Dec 21, 2016
186
I don't know if i chose the right place to post my thread, but im really struggling on this one. It's about CE amplifier. Im really sorry for asking such stupid question. But I really need help on that. I have searched on the internet about this stuff, but I hadn't any luck on finding what I needed.

I had solved many problems with CE amplifiers, but all these problems had almost everything known, but they have not any practical purpose for me. I want to learn how to design my own CE amplifier by myself.

How to solve a CE amplifier problem, with almost everything unknown? The only known things are the Vcc which would be in example VCC=6V, Vbe which is stable 0.7V, and the transistor in example 2N3904 with a β at a random value say 200 according to the datasheets. My goal now is to try to find:
1) the required resistors (except Re, which can be found with Ohm's Law),
2) the capacitors,
3) the Vin (this one i need to know in order to find the maximum voltage required to drive the amplifier without signal distortion on acoustic frequencies),
4) Ic, and Ib, (because Ie = Ic + Ib)
5) the voltages Vc, Ve, Vb and Vce, and
6) the Q point of transistor (this one i need to know because not every datasheet provide DC load lines.)

Current gains and Voltage gains would be highly appreciated, but not required.

It would be highly appreciated if you could provide only the formulas. I found many formulas on the internet, but all of them are very confusing, and does not exactly answer my question. These formulas provide everything necessary you should know, but still, there is some

For one more time, thank you.

PS At the end, i have a question on number 6). How can I know if my transistor works on operating region? How to make sure it is not working on saturation or cut-off region?

#### ericgibbs

Joined Jan 29, 2010
14,136
hi Paul,
Welcome to AAC,
It would help if you posted a diagram of the CE circuit, just add the components which you consider necessary and we can help you work out their values.

E

#### LvW

Joined Jun 13, 2013
1,337
How to solve a CE amplifier problem, with almost everything unknown? The only known things are the Vcc which would be in example VCC=6V, Vbe which is stable 0.7V, and the transistor in example 2N3904 with a β at a random value say 200 according to the datasheets. My goal now is to try to find:
Hi Paul - the starting point for designing a CE amplifier stage is the question:
What are your gain requirements? A gain of 10 or 25 ?
The answer to this question governs the selection of suitable parts.
And then, of course, the circuit should be shown or described:
* Which circuitry for base biasing?
* With or without signal feedback ? (Emitter resistor bypassed with a capacitor, yes or no?)

#### Ian0

Joined Aug 7, 2020
3,754
I got through most of that before I realised that you were not talking about CE Certification for an amplifier.
I can answer question 6 straight away. It is not saturated if the output voltage is above Vbe, and it’s not cut off if the output voltage is less than the supply voltage.

#### PaulEngineer

Joined Dec 21, 2016
186
Here is the circuitry

How to solve a CE amplifier problem, with almost everything unknown? The only known things are the Vcc which would be in example VCC=6V, Vbe which is stable 0.7V, and the transistor in example 2N3904 with a β at a random value say 200 according to the datasheets. My goal now is to try to find:
1) the required resistors (except Re, which can be found with Ohm's Law),
2) the capacitors,
3) the Vin (this one i need to know in order to find the maximum voltage required to drive the amplifier without signal distortion on acoustic frequencies),
4) Ic, and Ib, (because Ie = Ic + Ib)
5) the voltages Vc, Ve, Vb and Vce, and
6) the Q point of transistor (this one i need to know because not every datasheet provide DC load lines.)

Last edited by a moderator:

#### PaulEngineer

Joined Dec 21, 2016
186
Hi Paul - the starting point for designing a CE amplifier stage is the question:
What are your gain requirements? A gain of 10 or 25 ?
The answer to this question governs the selection of suitable parts.
And then, of course, the circuit should be shown or described:
* Which circuitry for base biasing?
* With or without signal feedback ? (Emitter resistor bypassed with a capacitor, yes or no?)
To your first question, i want to avoid any possible misunderstanding. Im really sorry to say, but I don't know what these values 10 or 25 is. The only thing I can answer, is that I want a high enough gain without distortion at the output.

The other answers to your questions i guess are provided in the circuitry

#### ericgibbs

Joined Jan 29, 2010
14,136
hi Paul,
You should have a target Gain in mind for the circuit and also the bandwidth required.
E
Update:
Lets choose say a gain of 25.
Bandwidth say, 10Hz thru 20kHz

#### PaulEngineer

Joined Dec 21, 2016
186
I got through most of that before I realised that you were not talking about CE Certification for an amplifier.
I can answer question 6 straight away. It is not saturated if the output voltage is above Vbe, and it’s not cut off if the output voltage is less than the supply voltage.
So you technically mean, that the transistor (in order to not work on Saturation region) should have an output more than 0.7V, and in order to be not cut-off, it shouldn't be less than 6V?

#### PaulEngineer

Joined Dec 21, 2016
186
hi Paul,
You should have a target Gain in mind for the circuit and also the bandwidth required.
E
Update:
Lets choose say a gain of 25.
Bandwidth say, 10Hz thru 20kHz
Sounds very good. 25 is very good gain. Bandwidth of 10Hz thru 20kHz sounds good too. I need to work it in a region of 20Hz to 20kHz

#### ericgibbs

Joined Jan 29, 2010
14,136
hi Paul,
Is this a homework/College assignment or just a personal study circuit.?

E

#### PaulEngineer

Joined Dec 21, 2016
186
hi Paul,
Is this a homework/College assignment or just a personal study circuit.?

E
Personal study

Edit: im generally new to the world of electronics. I've made some exercises on the very basics of electronics, and now I'm on the study of amplifiers. Well, it is really hard for me to understand this chapter (i have Malvino book 6th edition btw). I really need help on my question

Last edited:

#### LvW

Joined Jun 13, 2013
1,337
Sounds very good. 25 is very good gain. Bandwidth of 10Hz thru 20kHz sounds good too. I need to work it in a region of 20Hz to 20kHz
Paul - I think, it makes really no sense to calculate all the values and present the results to you.
For which purpose? You will learn nothing from this.
Hence two questions:
* Have you a rough idea how a transistor works and why it can be used as an amplifier?
* Do you know one of the possible forms for the voltage gain formula and the meaning of the transistor parameter gm (transconductance)?

#### PaulEngineer

Joined Dec 21, 2016
186
Paul - I think, it makes really no sense to calculate all the values and present the results to you.
For which purpose? You will learn nothing from this.
Hence two questions:
* Have you a rough idea how a transistor works and why it can be used as an amplifier?
* Do you know one of the possible forms for the voltage gain formula and the meaning of the transistor parameter gm (transconductance)?
1st question: Transistor can be used as a switch or an amplifier (at least that is how is presented on Malvino book). The second part of the question i can't answer correctly. But my opinion is that it can take a small amount of voltage as a signal, and amplify it at a higher level either to a second amplifying stage, or to the power amplifiers.

I have never heard of gm. But voltage gain is i think Ai=(Vo/Vi).

#### PaulEngineer

Joined Dec 21, 2016
186
I have quick searched for the BJT transconductance and found this one

Transconductance shows how sensitive collected current IC is with respect to the base emitter voltage VBE. gm=ICVT, where IC is the DC collector current at the Q-point and VT is the thermal voltage. This is the transconductance for a bipolar junction transistor (BJT).

The thermal voltage of BJT transistors, if i remember, is a stable value that is equal to 0.025V something, like that.

#### Ian0

Joined Aug 7, 2020
3,754
Gain = transconductance x load resistor

#### Audioguru again

Joined Oct 21, 2019
3,829
Hers is your simple transistor circuit with a load and with another emitter resistor to reduce the gain and reduce the distortion.
It has much more distortion than a simple circuit that uses an audio opamp.

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#### crutschow

Joined Mar 14, 2008
28,165
I want a high enough gain without distortion at the output.
Not possible to get no distortion with any amp, you just want to get it below the audible level.
The circuit you have would require a lot of negative feedback to get the distortion to an acceptable level (AG's circuit with the added un-bipassed emitter resistor helps).

If you just want to learn about a simple amplifier circuit, what you have is fine, but if you want to actually build a low distortion audio amp, you need a better circuit.

#### Audioguru again

Joined Oct 21, 2019
3,829
The single transistor circuit is too simple to have a gain of 25, a 6V supply, an output of 2V peak and low distortion. The gain can be reduced to 1 like a piece of wire then the distortion will be a little less.