Power Amp vs preamp topology

Discussion in 'General Electronics Chat' started by john_guitar, Jan 28, 2012.

  1. john_guitar

    Thread Starter New Member

    Dec 24, 2011

    I have the following problem that I cannot fully understand.
    In literature it is said that in a common preamp stage the topology for taking the output is as in attached figure top02.
    However, when it comes to Power amplification they always put the output transformer in the place of Ra (plate resistor) like in attached figure top01. Is it possible to create a topology like the attached figure top02 (which is similar to a preamp stage).?

    What are the differences between these two topologies??
    They say that power amps must amplify also current (not only Voltage amplification). If do we amplify the voltage, the current is amplified also (according to the resistance), isn't it?

    So, is it possible to have power amplification with topology in figure top01?

    Thanks in advance
  2. PaulEE


    Dec 23, 2011
    The difference between these two circuits lies in the output (as well as input, to an extent) impedances.

    The diagram with Ra provides voltage and current amplification (power gain) but has moderate output impedance.

    The diagram without Ra provides current gain but not voltage gain.

    The diagram without Ra is a circuit that is used to match a higher impedance source (such as a stage with Ra) to a lower impedance load (a transformer, as an example).

    These are analogous to common-emitter and common-collector amplifiers for BJT transistor stages.
  3. PaulEE


    Dec 23, 2011