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Transistor dc gain question ?

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crazyjohn

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How do you know which hFE value to use from a transistor data sheet in biasing a bipolar npn transistor in common emitter to get desired IC current when Ib current = IC/hFE is applied ?


Take a bc548 npn transistor for example how would you calculate the biasing current required on the base to get a desired current on the IC that = IB*hFE and what configuration do you use to achieve this ?
 

Normally you try to make a transistor amplifier tolerant of the wide variation in current gain. To do that you would include an emitter resistor which will be fully or partially decoupled and a base bias voltage divider which has enough current going through it that it isn't unduly affected by the base current.

Keith
 

Still a little confessed about this.

For example if i wanted to put a transistor in active mode so that i have a current of x ma to flow on the collector-emitter junction how do i know how much IB is needed.I look on a transistor data sheet i see stuff like minimum hFE and maximum hFE at different IC but no way of gauging exactly how much IB is needed or knowing the expected vbe.
 

but no way of gauging exactly how much IB is needed or knowing the expected vbe
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Yes, that's true. You would use a circuit, that can tolerate some amount of hFE variation by utilizing feedback. Most practical transistor circuits have it.
 

The value of hFE is a very unreliable design parameter. Read up on how to set up Common-base, Common-Collector and Common-Emitter gain stages to design amplifiers which are approximately independent of hFE.
If you want a x amount of current trough a particular branch in a circuit look up and design a circuit referred to as a constant current source. There are a few topologies of these constant current sources available. It might be what you are looking for.
 


Do most transistor amplifier circuits operate using an ac input signal and not dc signal?
 

No, they are used for DC amplification as well but the circuits need to be different. Do you have a specific application in mind? It is probably more common to use opamps at DC.

Keith
 

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