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The beginning of an audio amplifier

Discussion in 'Electronic Design' started by Joel Kolstad, Sep 17, 2003.

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  1. Joel Kolstad

    Joel Kolstad Guest

    Hi guys,

    I was wondering if you might take a look at this schematic here:

    It's the beginning of a class AB audio amplifier (ostensibly for around 50
    watts), and I had a few questions...

    -- Q2/Q3 and Q1/Q4 effectively make up 'compound' transistors, but this is a
    technique that I hadn't seen until recently. Are there any rules of thumb
    to choose the resistor sizes (R8 and R9)? I'm finding that making them
    smaller can significnatly reduce the quiessent current through the power
    transistors -- this occurs before a smaller resistors raises Vbe of Q3 or Q4
    to more closely match the biasing set up by D4 and D2. Hmm... perhaps I
    should choose R8/R9 to have the same current through them as D2/D4 do?

    -- I did start out just using two transistors in a Darlington configuration
    rather than the compound transistor approach shown. The drawback seemed to
    be the additional ~0.7 volts lost in 'head room.' Does this configuration
    have any additional benefits?

    -- How would you go about deciding the quiessent current through the
    R2/D4/D2/Q5/R3 leg? In classes I've taken on circuit design, usually it's
    dictated by frequency response, which doesn't seem to be much of an issue in
    an audio amplifier. I did discover some of the tradeoffs between that leg's
    gain and the output swing -- as gain is reduced (R3 becomes closer to R2),
    the negative-going output will saturate more quickly since there's a larger
    voltage drop across R3 and therefore the base of Q4 approaches Vee sooner.
    On the other hand, higher gains with smaller R3's make biasing Q5 more

    -- Any of suggestions would be appreciated. Thanks!

    ---Joel Kolstad
  2. Joel Kolstad wrote...
    You should worry about the Q5 current arising from the pullup resistor
    R2, which varies from a maxiumu for an output near Vee to nearly zero
    for an output near Vcc. The creates serious distortion and slew-rate
    problems that must be corrested by changing the design. At minimum R2
    must be replaced by a current source or by a bootstrap resistive tap.

    - Win
  3. Genome

    Genome Guest

    Sure, last time I was presented with a similar question on my HNC I ended up
    kicking my fucking copy of AoE around the fucking kitchen in disgust. I
    kicked the fucker in half. Then I re-wrote the fucking question and gave a
    proper fucking answer to the question. I'm still a stupid fucker, but I'm
    almost half less stupid than the fuckup who asked the question in the first

    And, Win, stop answering questions while you're pissed. I'm totally fucking
    pissed and still manage to retain a level of keyboard control that is

  4. Genome

    Genome Guest

    Oh.... Pissed means drunk. But you probably knew that.

  5. nospam

    nospam Guest

    You might try adding some gain in the output stage. A resistor from Q2
    collector to Q3 emitter and one from Q3 emitter to ground (and mirror to
    the -ve half).

    A gain of 2 (equal value resistors) will halve the voltage swing required
    from the input stage. Iooking like an additional load on the output it does
    waste some power.
  6. Joel Kolstad

    Joel Kolstad Guest

    Alright, I went home for dinner and picked up my copy of The Art of

    I thought I had worried about it by including Q5's degeneration resistor,
    R3? The distortoin is due to gm (of Q5) being a function of its collector
    current, as discussed on page 83, correct? Page 98 suggests that a current
    source is a better approach to fixing this problem than bootstrapping.

    It appears that what I'm calling the 'composite transistor' of, e.g., Q2 and
    Q3 is known as a Szikiai connection, discussed on page 95. Resistor R8,
    then, in my schematic is just the 'recommended' transistor to improve
    turn-off time and the value seems to be not too critical.

    Finally, I'm not following the example in the last paragraph on page 93: The
    temperature difference is 30 deg., so Vbe changes by 63mV. Great. But --
    assuming R3 and R4 have 50mA through them (preceeding paragraph) and
    therefore 50mV across them, won't we now have another 63mV->63mA across R3
    and R4, giving 113mA total, a 126% increase? (The book says 20%) -- What am
    I missing here?

  7. In addition to your wise choice of AoE, let me also recommend Douglas Self's
    "Audio Power Amplifier Design Handbook". It addresses the practical design
    issues of typical class-AB audio power amplifiers in excellent detail.
  8. Ted Wilson

    Ted Wilson Guest


    One of the major benefits of this type of Darlington arrangement is
    that it eliminates two Vb-e drops from the quiescent current
    definition and that the Vb-es removed are those of the transistors
    doing all the hard work and therefore subject to the greater
    temperature variations.

    Basically, you are attempting to define a voltage across R4 and R5 in
    series, which in turn defines the quiescent current in Q1/Q2. This is
    done by generating a voltage across D4 and D2 and applying this
    between the bases of Q3 and Q4. What is left across R4 and R5 is this
    voltage minus the Vb-e drops of Q3 and Q4.

    If you recall what you were taught about the accuracy of a small
    difference between two large numbers, you will see that significantly
    better than 2:1 improvement in quiescent current definition is
    achieved by reducing the number of Vb-es in the loop from four to two.

    Personally, I would substitute D4 and D2 with a Vb-e multiplier, using
    either a BD139 or a BD140, and thermally link this to Q3 and Q4.

    As Win points out, you need to define a constant through D4/D2, (or
    the Vb-e multiplier), so that, apart from dynamic considerations
    already mentioned, the voltage across them doesn't vary with output
    voltage, otherwise quiescent current will vary wildly with output


    Ted Wilson
  9. Ian Buckner

    Ian Buckner Guest

    You can also look at some of Doglas Self's work on his home site:

    (dipa = Distortion In Power Amplifiers.)

  10. Genome

    Genome Guest

    Whoaaa Neddy,

    The reason why I kicked your book around the kitchen was because the
    original question involved dogend like the circuit the OP presented.

    Some lazy arsed electronics lecturer asked us to design a piece of
    non-functional dirt single transistor amplifier with a gain of ten working
    off a 15V rail. Then he wanted us to add another gain of ten stage.... with
    an input signal of 1V pk-pk..... words such as.... 'on the basis of the
    transistors Hfe calculate the required base resistor to bias the transistor
    into its linear region and achieve a Gm of bloop bloop bloop.'

    I was lucky (?). I'd bought and read the AoE.

    First off I was annoyed at being asked to design a non-functional piece of
    dirt. Second I was seriously pissed (annoyed) because everyone else couldn't
    do it because the guys lectures were crap. Third I knew how to do a proper
    job because I'd read the AoE.

    What's the point of that? What a waste of time. I don't know why you and Mr
    Horrowitz bothered writing it. Arrggghhhhhh..... Kick kick kick.

    I gave it to one of the other people on the course and bought another one.
    I've been through another four copies since then. I'm a dickhead, I buy them
    and then give them away. Every time I end up working with a new keen
    engineer/technician they get a copy.

    Hope that clears up any misunderstanding...

  11. Genome wrote...
    Whew! Thanks, I feel beytter already.

    - Win
  12. Nice start, but

    Have you looked at the DC stability of this circuit?

    With no output capacitor, it's important to keep the output near zero volts

    But you're doing this by dead-reconing. The bias point for the first
    transistor is determined by R3 R6 and R7. If they're the regular 5%
    resistors, your output could easily be 5% off, which is a bit much DC. Also
    the bias point is a bit power-supply sensitive. Most amplifiers run off
    less than perfectly regulated supplies, so this might be a concern. And oh,
    a 2N3904 isnt specd at 60 volts is it?

    You might cheat and look at a 1964 GE transistor manual. There they gave
    examples of power amps made from germanium power transistors. Any bias
    circuit that keeps those transistors in a reasonable bias has GOT to work
    awfully darn well for silicons....
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