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# Basic Transistor Circuits questions

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While designing a transistorised circuit,say a voltage divider bias,an amplifier etc,should we select the transistor first and then decide the Q-point characterstics based on its datasheet or
we establish the q-point characterstics and then select a suitable transistor?
If condition2 is correct,then what values to select for Ic for say a CE voltage amplifier?deciding that Vce = 1/3 Vcc?leaving 1/3Vcc across Rc and 1/3Vcc across Re?

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Hi there

Usally you design a transistor stage based on its output demands,
voltage, current, power. Then you decide what transistor to use.
The emitter voltage is usally 5-10% of Vcc, but not always.

//Staigen

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But How does one decide Q point Ic i.e dc Ic?I find many students assuming Ic of 1 mA,just because its fairly a reasonable value.
So,when to use say,Ic of 1mA when to use Ic of 10mA or 100mA?

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Hi

What is Q? Quiescent? Usally Q means quality in electronics, used in resonant cirquits and filters! The inverse of Q is damping. The quiescent point depends on the output demands, if you need more than 1 mA out, than a quiescent current of 1 mA is a little bit low. Also, if you need an output peak to peak voltage of 10V, than a VCC of 9V is obviously not enough.

//Staigen

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• 3 weeks later...

A rule of thumb I learned in school:

Ure (voltage across emitter resistor) = 1/10 Uv (power supply voltage)
Uce (voltage across collector-emitter) = 1/2*(Uv - Ure)
Urc (voltage across collector resistor) = 1/2*(Uv - Ure)

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• 1 month later...

Perhaps your main concern might be noise. That will determine your choice of transistor right off the bat. Then you look at the noise characteristics of the transistor, and find out the collector current at which the transistor exhibits least noise.

What about the input impedance of the next amplifier stage? Your choice of resistors in an amplifier will be hugely influenced by this.

The source (say a microphone) will greatly influence your choice of base bias resistors. Your choice of emitter resistor will be somewhat limited by the transistor's gain, and bias current.

Perhaps you have to consider battery life. This means your design should draw as little current as possible, usually implying large resistances.

Any amplifier design is a compromise based on these kinds of considerations.

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