transistors

[prateeksikka]

hi kevin !
i too dont like non linearitities.
lower but linear beta is better for me.
hi audioguru!
that means
By changing emitter resistor we can change the gain of the transistor.
Whats the problem if we make extremely low Re?Do we have biasing problems?
or we may use high Rc?
Actually i wish to conduct a study on gain control of a transistor.How should i proceed and turn the knob for high gain?
;D

 

[audioguru2]

Whats is the problem if we make an extremely low Re? Do we have biasing problems?

You can't change a transistor's built-in Re, but you can bypass its emitter resistor with a capacitor for high gain. Without collector to base negative feedback, a transistor without an emitter resistor is not stable with transistor changes from one to another, nor with temperature changes. A common emitter transistor needs an emitter resistor.

or we may use high Rc?

Then you need a high supply voltage or a low current for it to be biased correctly. The output impedance will be high.

Actually i wish to conduct a study on gain control of a transistor.How should i proceed and turn the knob for high gain?

Use an emitter resistor so it works with different transistors and is temperature-stable. Then add a pot in series with a capacitor and connect them across the emitter resistor for your gain control. With the pot at low resistance, the emitter resistor will be bypassed for high gain. Notice the high distortion at high output levels. With the pot at high resistance then the capacitor doesn't do anything, and the gain is about Rc/Re, with low distortion. ;D

 

[SM2GXN]

Hi audiguru!

Is there any known good and easy way to calculate the gain when Re is bypassed by an capacitor, I know how to calculate the capacitor.
As you said the gain without bypassing Re is simply Rc/Re but when it comes to bypassing Re  :

Bjorn

 

[audioguru2]

Hi Bjorn,
Transistors have a built-in Re. It is calculated by dividing the transistor's hIE by its hFE. Both change when the transistor's current is changed.

Then the transistor's voltage gain is roughly the collector resistor (and the parallel load) divided by your calculated Re. Determining the voltage gain is rough because transistor amplifiers without negative feedback are not linear. On the collector voltage swing when the current is increasing, the gain is decreasing. A pure sine-wave input is distorted at the collector and looks like this on a 'scope:

View attachment 37340

 

[SM2GXN]

Hi audioguru!

Thank's for the reply.
What do think of this:
When the emiter resistor is bypassed the gain rise to very high values and the lowest frequency to be amplified depends mostly on the bypass cap, I'm thinking of audio now
As for the Ce I used to think of a divider which has a very low reactance for the frequency to be amplified at the bottom and in some way I'm back at the formula without bypass cap Rc/Re but Re is in parallel with the capacitor and thus making the reactance for the working frequency very low.
Without bypassing the emiter will follow the voltage change at the base, right?
Just my thoughts  ;D
Do you see my point?

Bjorn

 

[audioguru2]

Hi Bjorn,
If you bypass the emitter resistor with a capacitor having a very low reactance at the working frequency, then the transistor will have high gain and distortion.
You can adjust the gain by using two emitter resistors in series, with only one of them bypassed. The two resistors could be a pot. ;D

 

[SM2GXN]

Hi audiguru!

I know that the gain changes when the emitter resistor is bypassed and how it affect the frequency response of the amplifier. But you forgot to comment on my thoughts about the voltage divider
Can you figure out any parallell to the formula Rc/Re and the formula for a bypassed emitter resistor. What I meant was this, as the reactance gets smaller for the frequency to be amplified and Rc still got same value the total reactance the signal see is a very low emitter resistor though not pure resistive.
Then Rc/Xc I know it isn't that easy but is there any relationship at all?
It take some time for me to write all this en english and some of it can be difficult to understand, just let me know if there is anything you dont unerstand.

Bjorn

 

[audioguru2]

Hi Guys,
An expert at The Amplifier Institute:
http://dialspace.dial.pipex.com/prod/dialspace/town/pipexdsl/q/aqpl43/ampins/ampins.htm
agrees with my discussion about the beta of transistors:

BETA.
Beta is not a fundamental property of a BJT. Never design circuits that depend on beta. Unless of course you're making a transistor tester...

Here are some of the factors that affect beta. This should convince you that it is a shifty and untrustworthy parameter.

Beta varies with Ic.
First it rises as Ic increases, usually reaching a broad peak, then it falls off as Ic continues to increase.
Beta increases with temperature. This seems to be very little known. Most things, like leakage currents, get worse as temperature increases, so this makes a nice change.
Beta is lower for high-current transistor types.
Beta is lower for high Vceo transistor types.
Beta varies widely between nominally identical examples of the same transistor type.

 

[SM2GXN]

Hi audioguru!

Sorry for being stupid I must have been blind, all I asked for was in your post with the distortion.gif
Thing was that I've mixed emitter resistor RE and as you call it built in re so we can say that gain without bypassing RE is gain= RC/RE+re and with bypass cap gain=RC/RE.
Hopefully I got this right now  ;D
Thank you audioguru for once again clarifying in a easy way.

Bjorn

 

[audioguru2]

so we can say that gain without bypassing RE is gain= RC/RE+re and with bypass cap gain=RC/RE.

Hi Bjorn,
You almost got it right. The gain with RE bypassed is RC/Re. ;D

You can also use the emitter resistor's bypass cap as a treble-boost highpass filter. The frequency at which the capacitor's reactance equals the value of the emitter resistor the gain will be up 3dB, with 20dB for each decade of higher frequencies until the gain reaches RC/Re.

 

[SM2GXN]

;D yes my fingers made RE instead of re.
Do you mean voltage gain 6dB?
I finally found a link in my bookmarks that could be of interest to those who has little or no knowledge about transistors.
audioguru when you got time please check the link and let me know what you think, I find that the content of this link is a good tutorial.
Here it is: http://www.ee.nuigalway.ie/subjects/ee208/asd/notes/

bjorn

 

[audioguru2]

Do you mean voltage gain 6dB?

No, the gain is only 3dB because the reactance of a capacitor causes phase-shift. If the capacitor was a resistor with the same value as the capacitor's reactance then the gain would increase 6dB. ;D

 

[SM2GXN]

Yes ofcourse didn't think of that, thanks!
Once up on a time I could calculate most of this but as the years pass (getting older) some things just fade away ;D
Now days almost every experiment and new design has to be looked up in a book or by passing a new topic here  somtimes even the simplest circuit become a pitfall.

Bjorn