Transistor Designing?

[Electroenthusiast]

Voltage divider bias : I wanted to design a transistor with a specific voltage gain, for a constant input.
How should i design the circuit based on the value of voltage gain?
I know i can use a 'potentiometer in emitter side' to vary the gain of the amplifier.
What should i do now?....

 

[bychon]

The gain of a single bipolar transistor in common emmitter configuration is a little bit less than R collector divided by R emitter, if you haven't done something stupid like asking a single transistor to have a gain of 600 or carry 20 amps of current.

If you need a better answer, provide more facts, like which transistor, which configuration, how much current, how much voltage, what is driving the input, what is on the output side besides the collector resistor.

 

[Hero999]

There are several different ways of configuring a transistor amplifier.

We can only assume you're talking about common emitter because apart from being the most common, you mentioned altering the emitter resistor value to change the gain.

EDIT:

Changing the emitter resistor value might also upset the bias too much, you should couple the variable resistor via a suitable capacitor so it doesn't change the DC biasing.

 

[Electroenthusiast]

okay...
Common Emitter; what happens when i change both Re & Rc (Proportionally)...

 

[Hero999]

Changing both Re and Rc will not change the gain but it will increase/decrease the current and will alter the DC bias.

If you posted a schematic or told us what gain rage you require then we might be more helpful?

 

[Electroenthusiast]

Changing both Re and Rc will not change the gain but it will increase/decrease the current and will alter the DC bias.

If you posted a schematic or told us what gain rage you require then we might be more helpful?

i asked just for knowledge sake...
Thanks for the answers., it was enough to answer me.
Isn't there any Formula For it?

 

[12345678901234567890]

R1,R2,RE is used to set the bias voltage at the base, and the emitter current for the transistor, while the REcaps are switched in with different values of caps, and resistors, to change the AC gain of the stage variably.

By doing this the DC bias point is not affected.




And if you want the gain to be variable with a single dial, using a mosfet acts as a variable resistance to switch in a cap. to ground the signal, allowing the gain to be increased, the mosfet changes in resistance, as the RV pot. is dialed in.



With a little inginuity, RV. could be replaced with a LDR, or solar battery, a wind generator, a microphone, or any other sensor device, to change gain with respect to an analog input.

A input signal to the base to be amplified, and a continuous variable gain to the stage could make a pretty interesting project.

 

[Space Varmint]

The gain is built in. It's marked as the "beta" of the transistor.

 

[audioguru]

The gain is built in. It's marked as the "beta" of the transistor.

Beta is the current gain of a transistor that is not its voltage gain.
The circuit "tries" to use a Mosfet to vary the emitter resistance to change the voltage gain of the transistor circuit. But the Mosfet doesn't work because its source is in series with a capacitor instead of being connected to ground. The capacitor should be in series with the drain.

 

[Space Varmint]

You also have DC gain and AC gain. The beta determines DC gain primarily and hfe is significant in AC gain. So higher frequency transistors have more AC gain but if operated way over frequency then they are very prone to oscillation.

 

[audioguru]

hFE is DC current gain.
hfe is AC current gain.
Neither is the voltage gain of a transistor circuit.

The voltage gain of a transistor circuit is Rc/(Re + the transistor's internal Re). The Rc includes the load resistance which might be in parallel. The input of a common-emitter transistor circuit has a fairly low resistance so the drive signal must be from a very low resistance for max voltage gain.
A high frequency transistor has low capacitances so it can have plenty of voltage gain at high frequencies.

 

[Hero999]

hFE is DC current gain.
hfe is AC current gain.

I often get that confused.

I know the convention is to use lower case for AC and upper case for DC but I can never remember which letters to capitalise.

What does hFE actually stand for anyway? I know it means the current gain of a transistor but what does each letter stand for?

 

[Space Varmint]

I often get that confused.

I know the convention is to use lower case for AC and upper case for DC but I can never remember which letters to capitalise.

What does hFE actually stand for anyway? I know it means the current gain of a transistor but what does each letter stand for?

Oh yeah, I meant the "ft" frequency transition. That is significant to the AC gain.

 

[12345678901234567890]

I often get that confused.

I know the convention is to use lower case for AC and upper case for DC but I can never remember which letters to capitalise.

What does hFE actually stand for anyway? I know it means the current gain of a transistor but what does each letter stand for?

If I am remembering this correctly,

HFE is used in H- parameters (hybrid model in common emitter mode)

It is the Forward current transfer ratio.

HFB is used to show the transfer ratio with a common base configuration. as in 25 / IE (ma)

 

[12345678901234567890]

Here is the circuit refined to actually work.

Without the drain resistor RD the mosfet causes distortion on the output waveform UNTIL the mosfet is turned on harder to make it work more linear.

By putting RD in there causes the mosfet to be biased on also before the mosfet begins to have influence on the emitter circuit, it is conducting more heavily with a higher input to its gate. so the small nonlinear changes are swamped out, the output waveform remains at zero change until a large enough linear signal from the mosfet is felt at the drain to have a much more linear affect on the emitter lead.



Sorry ,I just went back and read your very first post of this thread,
you were asking how to design for a specific gain, not how to vary the gain.

I will still leave this schem. post here, just in case you find it of interest.

 

[audioguru]

A Jfets that is used to limit the output of a Wien Bridge oscillator needs to have a very low signal level (less than 100mV p-p) at its drain to reduce non-linear distortion. The Jfet has half of the signal at its drain fed to its gate to reduce even-harmonics distortion.

 

[Hero999]

If I am remembering this correctly,

HFE is used in H- parameters (hybrid model in common emitter mode)

It is the Forward current transfer ratio.

HFB is used to show the transfer ratio with a common base configuration. as in 25 / IE (ma)

Thanks for that, I'll try to remember it.

Where did you learn that?

 

[12345678901234567890]

Thanks for that, I'll try to remember it.

Where did you learn that?

Hi,
about 25 years ago, I took a corespondance course from "CLEVELAND INSTITUTE OF ELECTRONICS", (CIE).

 

[Space Varmint]

Hi,
about 25 years ago, I took a corespondance course from "CLEVELAND INSTITUTE OF ELECTRONICS", (CIE).

I remember them. I almost did it myself but I took NRI (Nat'l Radio Institute). It was pretty intense. I wish I still had my books. They had some stuff I've never seen since, like calculating out ladder networks in attenuators, filters and equalizers.