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Transistor amplifier -- base voltage?

Discussion in 'Electronic Basics' started by Ken, Jan 24, 2004.

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  1. Ken

    Ken Guest

    I am not an electrical engineer and am trying to figure out what
    voltage to put on a Motorola mobile speaker squelch lead. My research
    has taken me to the [low] limit of my comprehension.

    The lead ultimately reaches the base of a transistor amplifier set up
    as a "common emitter." The collector is connected to a number of
    sub-circuits that apparently effect the squelch function. There is a
    table on the schematic that states the following for the subject
    unsquelched: base=0.6, collector= 0.4
    squelched: base=0.0, collector= 7.5

    I take this to mean that a voltage of .6 volts at the base will open
    the squelch and a voltage of zero will close the squelch.

    My most convenient voltage source is readily adjustable from 0 to 1.2
    volts at the base.

    (1) Will it harm a transistor amplifier if 1.2 volts is applied to the

    (2) What will happen to the collector voltage if the base voltage is

    (3) Is it important that I change my source so that the voltage at the
    base will never exceed 0.6V?

    Ken C

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  2. Tim Dicus

    Tim Dicus Guest

    Not normally. Most transistors are rated to take at least 3 volts at the base junction. There are exceptions I understand. Check
    your datasheet carefully!
    It will normally go into a state called "saturation". The collector-emitter junction will pass as much current as it is rated to
    No. Just limit the current to the base with a bias resistor or similar device. My rule of thumb when using a transistor as a switch,
    is to use enough base current to send the transistor into saturation, but no more. I have found through trial-and-error that it
    takes longer for a transistor junction to come out of saturation the higher the base current was.

    Just so you know why sometimes I refer to "trial-and-error", when I got started in electronics, transistors were mentioned in my
    textbooks as "future development". That makes me feel old!

    Hope that helps,

  3. DarkMatter

    DarkMatter Guest

    Telling us that here succinctly dates you as such as well.
  4. John Larkin

    John Larkin Guest

    Likely yes, certainly if your source can supply a lot of currrent. Use
    a series resistor, 100 ohms maybe between your source and the base, to
    limit the base current.
    The transistor will be probably fried, so it could be anything
    depending on things. When you apply the 1.2 through a resistor, the
    collector voltage should drop to near zero.
    Resistor does that for you.

  5. Ian Bell

    Ian Bell Guest

    Almost certainly. You need to feed the base from your voltage source via a
    resistor. If you voltage source is 1..2 volts then I would suggest a 1000
    ohm resistor should do the job.

  6. Ken

    Ken Guest

    Thank you. All very helpful.

    Ken C

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  7. Ken

    Ken Guest

    To save bandwidth I did not include more details from the schematic.
    The squelch lead passes through a 1K resistor before it arrives at the
    base. 0.6 volts is the voltage after the resistor. I figure that the
    voltage at the lead that gives 0.6 volts at the base is no more than 5
    volts. My convenient source is actually 12VDC, which I figure will
    attenuate to no more than 1.2 VDC after the resistor at max base
    current. Unfortunately, my graph for base current stops at 0.6 volts
    base voltage.

    I will start experimenting with a 1 VDC source before the resistor and
    gradually increase it to 12VDC, measuring base voltage. When I get to
    0.6, that (plus maybe a little more) is where I will stop.

    Ken C

    (to reply via email
    remove "zz" from address)
  8. Gareth

    Gareth Guest

    The voltage after the base resistor will not double if the voltage
    before it doubles. This is because the base current is exponentially
    related to the base voltage, i.e. a very large increase in base current
    will only increase the base voltage by a small amount. It is therefore
    a reasonable approximation to assume that the base voltage will not
    exceed 0.6V.

    You can now calculate what resistor you need to keep the base current
    the same when you use a 12VDC in place of 5VDC.

    If in the original circuit 5V was applied to the base via a 1K resistor
    the current would have been:

    Base current = (5-0.6) / 1000 = 4.4 mA

    to get the same base current with a 12V supply you would need:

    R = (12-0.6) / 4.4 = 2.59 K ohms.

    So if you add a 1.6 K ohm resistor between the 12V and the existing 1K
    resistor you should get the same base current as before.

    If think the original voltage may have been a bit less than 5V try a
    larger resistor to start with.
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