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We know that bjts operate in the principle of base current controlling the collector current. If there is more base current, then more collector current flows.

Now going to the input part of a amplifier, when designing the input side (preamplifier), do we need to consider the base currents, Max Ic collector current and stuff during deaign phase (computing bias calculations,etc) since what were concerned of is only the input voltage?

As far as i know, what the pre amplifier cares about is the input voltage and output voltage and it doesn't care about base current, Max collector current, and the like(anything that has current).

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closed as not a real question by Anindo Ghosh, Olin Lathrop, Leon Heller, Dave Tweed, Kaz Nov 28 '12 at 0:30


A BJT is a current driven device, so any BJT based device "cares" about the currents, I would say. – Anindo Ghosh Nov 24 '12 at 12:12
The BJT is modelled as a transconductance device; the collector (output) current is controlled by the base-emitter (input) voltage. The output current through a load produces an output voltage producing a voltage amplifier. Typically, the actual input voltage does not appear across the base-emitter junction but rather, but rather, across the base-emitter junction in series with some resistance. – Alfred Centauri Nov 24 '12 at 12:29
This question is nonsensical. A audio preamp deals with voltage signals on its input, but that doesn't mean you can ignore currents inside the amp, of course. – Olin Lathrop Nov 24 '12 at 13:55
@OlinLathrop in what way can we utilize current inside the ""preampflier itself"", afaik, we only deal with voltage amplification in pre amp, yet current comes into a great play in power amp since we want current amplification. – WantIt Nov 24 '12 at 13:59
up vote 1 down vote accepted

You must give it some consideration. As a first step, here's how it might work.

Specify the input voltage you need to give full output (a typical value is 1V rms) - this lets you work out the voltage gain of the amp, but that's another matter. Also specify the input impedance (10kilohms is a typical value). Divide, and you have the input current (in this case, 0.1ma rms). Now, the AC component of the BJT base current cannot exceed this, and really should be much lower (or the amplifier will be non-linear, i.e. have poor distortion). A suitable value for the base current might be something like 0.01ma, until we have better information (like, the distortion is worse than we want).

Now you can combine this info with the current gain of each stage of the amp, and see if you can meet this requirement AND supply the output current you need, or if you need more gain. If the first stage has a current gain of 100, then it can supply 1 ma to the next stage, and so on.

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