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AC Coupled amplifier

  1. Nov 5, 2009 #1
    Hey guys.

    I want to design an AC coupled amplifier, to be powered by a 12V supply. The input voltage should be less than 30mV and the gain of the circuit should be 24dB. The circuit has 2 stages with the gain of the 1st transistor amplifier being used to amplify the signal going into the 2nd transistor.

    I already know how the circuit should look but I need to ensure that the input impedance is greater than 1.2million ohms and the output impedance less than 180ohms.

    From the circuit below, does anyone know what resistor values I should use for this criteria?


    Thanx in advance.
  2. jcsd
  3. Nov 7, 2009 #2


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    You are barking up the wrong tree. You will need alot more components with a bunch of negative feedback to achieve an input impedance that high with that much gain. That would be my approach anyway. What is wrong with using several op-amps?
  4. Nov 7, 2009 #3


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    Agreed. It is pretty hard with discrete analog devices. The gain of transistors can vary by as much as 3 to 1, so you end up making it and adjusting it just to make it work.

    The "gain" specified in dB is actually a fractional voltage gain due to the different input and output impedances. If someone wants a voltage gain of 16 they should just say so, not try to dress it up with dB figures.

    The gain of the first stage is approximately RC / RE and the input impedance is approximately hfe * RE. But the bias resistors appear in parallel with the inputs and the loads for both stages.

    Both of these calculations force very large values of RC and RE so the collector current becomes very small and the base current gets microscopic.

    If I had to use those components, it might be worth using the FET as the input device.
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