# Resistors in an amplifier

Discussion in 'General Electronics Chat' started by Zanac-X, Apr 14, 2012.

1. ### Zanac-X Thread Starter Member

Dec 23, 2011
51
1
hey to all

can anyone explain to me in details about the use of resistors in an NPN transistor working as amplifier , R1,R2 in voltage divider in (B) ,RC ,RE

if i have only a transistor used as amplifier with one resistor connected between Vcc and B , and RC resistor (Between Vcc and C) ,whats the difference between the first and second circuit

any help is much appreciated

thanks

2. ### MrChips Moderator

Oct 2, 2009
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A picture is worth a thousand words. Post a circuit diagram.

3. ### Zanac-X Thread Starter Member

Dec 23, 2011
51
1
maybe this will help explain what i want

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4. ### Audioguru New Member

Dec 20, 2007
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Case 1 has a voltage divider feeding the correct voltage to the base of the transistor. There is an emitter resistor so that any change in the current gain or base-emitter voltage is cancelled.

Case 2 has only one resistor feeding base current. Since transistors (even with the same part numbers) have different current gains and different base-emitter voltages then it is cutoff or saturated and probably cannot amplify without severe distortion.
I was taught to NEVER EVER bias a transistor like that unless the transistor is a simple switch.

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5. ### MrChips Moderator

Oct 2, 2009
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Case 1 is the preferred way to bias a transistor. Re provides negative feedback and hence gives better temperature stability.

As audio says, use Case 2 only when the transistor is used as a inverting logic switch .

6. ### Adjuster Well-Known Member

Dec 26, 2010
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300
If you set up Case 2 with Rb/Rc >>β, it can be possible to get some linear operation, from the arrangement, but this is really not very satisfactory.

The collector current will be quite unpredictable, being very dependant on β, which is normally a quite variable quantity.

In fact, a crude indication of β can be made with a circuit of this kind. If Rc is replaced with a mA meter, and perhaps a small series resistor for safety, and Rb is chosen to give a suitable base current, provided the collector voltage drop is not too large, the collector current roughly indicates the gain.