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How to make a desired voltage in a transistor CB amplifier?

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michael 1978

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Can you help with a transistor common base amplifier? i know how it works, but i dont know how to make desired voltage, for example i want 50mA of 80 of 100 mA desired voltage gain
thnx.
 

Re: Hi help transistor CB amplifier

"e i want 50mA of 80 of 100 mA desired voltage gain" Voltage gain is a number - not mA . In a common base the same AC current flows through the transistor, so with a low input resistance, Re and a high output resistance, the output resistance is normally taken as the collector load. So voltage gain = collector load/Re.
Frank
 

Re: Hi help transistor CB amplifier

hey Frank thnx for reply, Frank i will like to ask you, if you have time for me? How can i design one amplifier with gain of 50mA, let say input is AC 0.1mA RMS, i mean Common Base Amplifier, And i don't understand "voltage gain = collector load/Re."
Re is Emitter resistance?
 

Re: Hi help transistor CB amplifier

A CB amplifier has current gain = 1.
This means:
a) If input current is 0.1mA RMS, then output current is 0.1mA RMS.
b) If you want output current = 50 mA, then you need to give input current = 50 mA.

michael 1978 said:
...amplifier with gain of 50mA...
Click to expand...
That doesn't make sense.

Voltage gain is just a number (no units).
Voltage gain = output voltage / input voltage

Current gain is just a number (no units).
Current gain = output current / input current

michael 1978 said:
Re is Emitter resistance?
Click to expand...
Yes
 

Re: Hi help transistor CB amplifier

no that is not true, i have one example of cb amplifier, where
Input voltage is 0.5mV and
Output voltage is 50mV
 

Re: Hi help transistor CB amplifier

michael 1978 said:
no that is not true, i have one example of cb amplifier, where
Input voltage is 0.5mV and utput voltage is 50mV
Click to expand...
That is a voltage gain of 100. Its current gain is 1.
Don't you know what the difference is between voltage and current?
 
Re: Hi help transistor CB amplifier

voltage is presssure, of elecical motive force, aand curren simpel, the movmenet of electrons, but i ask how to bias cb amlifier there are grounded all emitter, base collector, so i dont know how much voltage to give to emitter and collector
 

Re: Hi help transistor CB amplifier

How did you select common base? What is the signal source? Can the signal source drive the low input impedance of the common base transistor?
 
Re: Hi help transistor CB amplifier

michael 1978 said:
i ask how to bias cb amlifier
Click to expand...
For a simple circuit with a single supply, we want:
a) Collector voltage = about Vcc / 2
b) Emitter voltage = much lower, maybe 0.5V to 1V
Remember base voltage = emitter voltage + about 0.7V

What to do:
1) Choose emitter voltage Ve and collector current Ic.
2) Bias the base to the correct voltage with resistor voltage divider between Vcc and ground.
3) Calculate emitter resistor = Ve/Ic
4) Calculate collector resistor = 0.5 * Vcc / Ic
 
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Re: Hi help transistor CB amplifier

i want to know how to make desired voltage gain, for example 50mA , of 80mA of 100mA,
what i have to do first, where to start...thnx, because all are grounded base emitter and collector
 

Re: Hi help transistor CB amplifier

michael 1978 said:
i want to know how to make desired voltage gain, for example 50mA , of 80mA of 100mA,
what i have to do first, where to start...thnx, because all are grounded base emitter and collector
Click to expand...
Again you DO NOT UNDERSTAND voltage and current. 50mA, 80mA and 100mA are currents, not voltage gains.
An input voltage of 0.5mV and an output voltage of 50mV is a voltage gain of 100.
An input current of 0.5mA and an output current of 50mA is a current gain of 100.

You also do not understand transistors because when the base, emitter and collector are all grounded then it will not have gain of anything (a gain of weight).
Voltage gain is produced by a common-emitter transistor that can also produce current gain.
Voltage gain is produced by a common base transistor that has no current gain.
Current gain is produced by a common collector transistor that has no voltage gain.
 
Last edited:

Re: Hi help transistor CB amplifier

VOLTAGE gain is produced by a common base transistor, that has no current gain is I think what you meant to say?

73 Dan.
 

Re: Hi help transistor CB amplifier

I know the input is Emitter Base and Output is Collector Load, but my problem is, i want to design Common Base Amplifier with desired Voltage gain for example 50mV, 70mV of 100mV how can i design amplifier? thn x
 

Re: Hi help transistor CB amplifier

@Michael:

Everything you have written in this thread is complete nonsense.

Please read what others have written.

We cannot help you if you do not read anything we write.
 

Re: Hi help transistor CB amplifier

Oh Sorry you are right, i thought mV sorry, ye like i say a Common Base Amplifier with desired Voltage Gain of 50mV, 80mV, 100mV, how can a design a amplifier with voltage gain for example of 80mV
I know input is Emitter Base and Output Collector Base
Where to start how much supply do give to Emitter and Collector, i relly dont know can you help me please thnx

- - - Updated - - -

Why nonsenese I ASK FOR HELP OK

- - - Updated - - -

Why nonsenese I ASK FOR HELP OK
 

Re: Hi help transistor CB amplifier

There is no such thing as a voltage gain of 80mV. Voltage gain is simply a number. Maybe you want an AC voltage gain of 80.
But the voltage gain is affected by the source impedance and by the load impedance like this:
 

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Re: Hi help transistor CB amplifier

Yes in fact i want AC voltage gain 80, but please can you tell me the formula? how you find excatly AC voltage gain of 80, wich step you take, what you did first, CAN YOU TELL ME STEP BY STEP PLEASE THANX
 

If the source impedance is lower than RE, Re and the low input impedance of the transistor then the voltage gain is higher than you think.

If the source impedance is higher than a few ohms then it is in series with RE and Re which reduces the gain.

Somebody might show the calculation but why bother unless it is homework for school.
 

Yes i understand i dont want to bother nobody....
but i need help to know the formula for desired voltage gain, like you show me a circuit with gain of 80mV, where you start, what was the first step, please let me know...you make a circuit with 80mV gain, let me know also the rest maybe tomorow:
 

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