When BJT are used as amplifier and when FET are used as amplifier

Discussion in 'General Electronics Chat' started by jchawla42@Gmail.com, Jan 8, 2012.

  1. jchawla42@Gmail.com

    Thread Starter New Member

    Jan 8, 2012
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    Sir I have a question kindly answer it out...
    When BJT are used as amplifier and when FET are used as amplifier
     
  2. bertus

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  3. Adjuster

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    On the whole , BJTs are much more used in amplifiers, or at least historically they have been. This is partly because BJTs were commercially available long before FETs were, but the BJT has other advantages, such as more predictable voltage/current characteristics, and producing a large current change for a small input voltage change. This makes it easier to create linear wide bandwidth amplifiers using BJTs

    The advantages of BJTs have however been somewhat eroded by progress in FET technology, and FETs also possess some particular advantages. The most important of these is their high input impedance (their gate currents are extremely low, often quite negligible). BJTs by comparison have lower input impedance, and require appreciable base currents. One downside of the FET's high input impedance is that it can be more easily damaged by static: only a small charge is needed to raise a damaging gate voltage.

    FET amplifiers are therefore particularly useful where a high input impedances / low input currents are needed. Most of the lowest input current operational amplifiers are of this type.
     
  4. crutschow

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    To supplement Adjuster's excellent post, (MOS)FETs are often used in circuits that have a significant power output such as audio and servo amps since, at high currents the high required base current of BJTs can significantly reduce efficiency. They are also used in the power stage of switching power supplies because they switch much faster than BJTs, which increases efficiency.
     
  5. spinnaker

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    And not to hijack the OPs thread but for us electronically challenged folks does this mean the use of an FET in an amp mean less need for a preamp?
     
  6. Adjuster

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    I don't really think so - the reasons usually advanced for separating pre and power amplifiers have more to do with benefits and disadvantages of different parts of the system being close to each other, or well separated.

    Thus a power amplifier may be better close to the loudspeaker it drives, since long cables extending the low-impedance speaker circuit may degrade the signals. On the other hand, the sensitive circuits of a pre-amplifier may suffer from interference in proximity to a power amplifier and the heavy power circuits that supply it.

    A pre-amplifier may also be better put near any low-level signal sources like microphones or vinyl record players, so that feeble signals do not have to pass through long cables.

    A fairly long interconnect between the pre-amp and power-amp should not introduce too much degradation, as the level is reasonably high, and works at a moderate impedance. These arguments are not really altered by using a particular input device.
     
  7. crutschow

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    The primary advantage of a FET in an amp is it's high input impedance so they are good for high impedance sources like some microphones. The lowest noise transistors are actually BJT which are good for lower impedance sources.
     
  8. spinnaker

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    So how did the handle this pre-FET? Just through additional circuitry to create a high impedance input tot eh amp?
     
  9. Adjuster

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    Much earlier amplifiers used vacuum tubes, which also typically have very high input impedances!

    Bipolar transistor amplifiers can also have relatively high input impedances too though, particularly in certain negative feedback configurations, such as the emitter follower (also known as common collector).

    A high-gain BJT used as an emitter follower, working at a at low current, and with a reasonably big load impedance, perhaps with bootstrapped bias, can get up to the few megohms input impedance that is about the limit required for most audio purposes. (High enough for say an old-style piezoelectric phono pickup or microphone, maybe not so easily high enough for the type of condenser microphone that does not contain an integral FET. )

    Similarly, a bipolar op-amp can have its already fairly high input impedance raised by feeding the input to the non-inverting input, with negative feedback applied to the inverting input.
     
  10. studiot

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    Not at all if you you want tone controls, balance controls, filters etc etc.
     
  11. Adjuster

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    That's a good point. I had thought the OP was referring to the argument for a separated preamplifier versus an integrated amplifier. Even the latter can normally be divided schematically into power amplifiers and a preamplifier section which provides high-sensitivity inputs and controls.

    While having FETs may make it easier to make a power amplifier with a hi-Z input, it does not make much difference to the requirement for at least a preamplifier section in an integrated amp.

    A FET input might for instance let you run a basic power amp straight off an old high-output crystal pickup cartridge (several hundred mV into 1MΩ, but at about 9 grammes force tracking, not so good for your precious vinyl). Gets you a cheap record player with no controls, but that's not a substitute for a full amplifier setup.
     
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