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NPN Common Emitter Bias

Discussion in 'Electronic Design' started by dgc, Jan 4, 2006.

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  1. dgc

    dgc Guest

    Can an NPN transistor in a common emitter configuration be safely biased for
    standing DC current WITHOUT having an emitter resistor in the circuit, i.e
    emitter directly tied to ground? I'm trying to accomodate a 12 volt peak
    swing on the collector with only a 12 volt supply available. Any emitter
    resistor obviously lowers the peak voltage the collector can handle, while
    remaining linear, so I am trying to avoid using one.

  2. You can use collector voltage feedback, instead.
  3. dgc

    dgc Guest

    Thanks John. I have seen collector voltage feedback modeled in a text I am
    reading. However, just for my education, could a transistor be biased for
    standing DC current with a direct ground connection on the emitter, without
    damaging the transistor.
  4. Tim Wescott

    Tim Wescott Guest

    It can be done, but it gets complex. "Radio Frequency Transistors --
    Principals and Practical Applications" by Dye and Granberg has some
  5. If the transistor is kept at a fairly constant temperature, and the
    bias is carefully set, the transistor is unlikely to be damaged, since
    in a class A operation, the worst case power dissipation occurs at
    zero signal. The real problem is whether the bias point will stay
    centered well enough for the required signal swing without clipping.

    If the temperature changes more than a few degrees, all bets are off.
    Transistors are pretty good thermometers.

    A simple collector voltage bias involves connecting the base divider
    to the collector, instead of to the supply, with the top resistor
    value dropped to account for the lower voltage across it.
  6. Pooh Bear

    Pooh Bear Guest

    Not easily.
    Any objection to an IC if it's small signal ?

    What's the application ?

  7. Pooh Bear

    Pooh Bear Guest

    Lower value resistors reduce the beta ( current gain ) sensitivity too.

    Of course this configuration applies negative feedback - No idea if that's an
    issue for the OP.

  8. Andrew Holme

    Andrew Holme Guest

    I've seen a circuit where the NPN is biased using a PNP transistor.
    collector to the PNP emitter; with a potential divider setting the PNP
    base voltage; and the PNP collector feeding bias to the NPN base. I
    think it was a wideband RF buffer stage in a Marconi signal generator.
    I'll check the service manual when I get home tonight.
  9. Fred Bloggs

    Fred Bloggs Guest

    This can be done any number of ways but your question cannot be answered
    without knowing the frequency range of the signal to be amplified, the
    required gain of the circuit, the loading, and the source
    characteristics. Local feedback is not going to cut it since the
    transistor gain goes to zero at cutoff and saturation.
  10. Tim Wescott wrote...
    I don't have that book, but many wideband linear RF amplifiers I've
    worked on used a current-sensing resistor in the collector RF path,
    with a little servo circuit to establish the average value of the
    base current. This is easy to do with just a few transistors. The
    sense resistor need not drop more than 200 to 400mV.
  11. Ian Bell

    Ian Bell Guest

    Yes, but you need feedback of a different sort. My ascii art is crap so I
    will describe one type of circuit. Collector resistor to Vcc, emitter to
    gnd. Resistor from collector to base. Resistor from base to gnd. Collector
    volts is essentially vbe multiplied by the collector/base base/gnd resistor
    ratio. This sets both ac and dc gain. You can separate ac gain by slitting
    the collector/base resistor into two equal parts and decoupling the
    junction to ground.


  12. Jim Thompson

    Jim Thompson Guest

    AND: TC of output DC bias point = -2mV/°C * DCGAIN

    ...Jim Thompson
  13. Andrew Holme

    Andrew Holme Guest

    They used lots of variations: always with a resistor between PNP collector
    and NPN base; often with a large capacitor from PNP collector to ground;
    only sometimes with a capacitor from PNP emitter to ground; and sometimes
    with a small NPN emitter resistor. None are anything like rail-to-rail
    output. Sometimes there's a diode in series with the potential divider -
    presumably for temperature compensation. I also found an example where they
    used an NPN instead of the PNP, but enough said.

    This is from a 500-900 MHz first LO drive circuit:

    .------------------------o------- +10V
    | |
    | .-.
    | | | 100-ohm
    | | |
    .-. '-'
    | | |
    | | .-------o-------o
    '-' | | |
    | | | |
    | |< --- |
    o------| --- 1n C|
    | |\ | C| L
    .-. | | C|
    | | | === |
    | | .-. GND |
    '-' | | o-------- out
    | | | 1k5 |
    === '-' |
    GND | |
    | |/
    in --------------o-------------|
    created by Andy´s ASCII-Circuit v1.24.140803 Beta
  14. Ian Bell

    Ian Bell Guest

    Quite right, and probably the reason it is rarely used. In simple
    applications where temperature range is limited it may not be a problem.
    With a 12V supply and a 6V collector voltage the temp co is about
    20mV/degree which over a 20 degree range is just 400mV.

  15. John Larkin

    John Larkin Guest

    Well, just run the low side of the bottom resistor to a negative
    voltage. That's a great way to bias gaasfets, too.

  16. Ian Bell

    Ian Bell Guest

    I suspect if the OP had a negative voltage available he would want the
    output to swing all the way down to it ;-)

  17. Andrew Holme wrote...
    The circuit has three problems, all easily fixed. First,
    the bypass cap should go across the sense resistor, etc.,
    because the goal is to get a constant base-bias current
    to the RF transistor, unaffected by supply fluctuations,
    RF currents, etc. Second, we don't want the servo-loop
    gain too high at high frequencies, so some PNP emitter
    resistance is in order. Third, we'd like to reduce the
    wasted voltage drop across the sense resistor, yet avoid
    tempco issues with the PNP, so we need a tempco-matching
    transistor. Here's the result of these changes.

    .. .---------------------+----+------- +Vcc
    .. | | |
    .. R1 _|_ Rs
    .. 250mV --- 200mV
    .. | | |
    .. v\| .--/\/\--+----+
    .. |---, | 50mV |
    .. /| | |/v |
    .. +-----+----| C|
    .. | |\ C| L
    .. R2. | C|
    .. 5mA | |
    .. etc | |
    .. | | +-------- out
    .. GND | |
    .. | |/
    .. RF IN------+-----------|
    .. |\v
    .. |
    .. ===
    .. GND
  18. Guest

    Gee, I thought I'd invented this scheme, used in a 920MHz RF power
    amp way back when ;-) But heck, I was a noob, hacking together some
    stuff for the project of the day.

    Two quibbles: the inductor must be bypassed, else the added series
    resistance can spoil its 'Q', and, more importantly, you'll get
    undesired feedback around Q1, collector-to-base through the
    current-sensing transistor Q2.

    Secondly, Andrew's resistor in the collector of biasing/current
    sensing transistor Q2 is a good idea, to avoid loading the base of RF
    amplifier tranny Q1.

    James Arthur
  19. Guest

    Winfield Hill wrote:
    Here's a version that makes sure the inductor is bypassed at RF
    frequencies, yet, per Win, avoids AM modulation of the bias from power
    supply noise:

    .. .---------------------+----------+------- +Vcc
    .. | C1 | |
    .. R1 100n _|_ Rs
    .. 300mV --- 200mV
    .. | R3 | R4 |
    .. v\| .--/\/\--+--/\/\----+--------.
    .. |---, | 50mV 50mV | |
    .. /| | |/v | --- C2
    .. +-----+----| C| --- 1n
    .. | Q2 |\ C| L1 |
    .. R2. | C| ===
    .. 5mA .-. | GND
    .. etc | | R5 |
    .. | | | 1k +------------ out
    .. GND '-' |
    .. | Q1 |/
    .. RF IN------+-----------------|
    .. |\v
    .. |
    .. ===
    .. GND

    James Arthur
  20. wrote...C2 = 1nF ??
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