Help with understanding volume control circuit

Discussion in 'Homework Help' started by JosSchr, Feb 17, 2016.

  1. JosSchr

    Thread Starter New Member

    Feb 17, 2016
    Hello, everyone!

    I am a student of electronics and I have become pretty good at analyzing circuits, but I still have problems with understanding practical circuit designs. Here is a circuit I would like to understand more. It's a circuit to control the volume of a loudspeaker.


    I understand the basic principle. The op-amp is an extended inverting amplifier with potentiometer-controlled amplification that amplifies the square wave created by the leftmost source. After the op-amp there is also a high-passfilter (?) to filter out any mechanical noise. So here are my real questions:
    • Why are there two PNP-transistors connected between the op-amp feedback loops and the high-passfilter?
    • What does the positive feedback loop do? It looks like a low-passfilter but why do we need that?
    • What are the capacitor and resistor connected in series in between the square wave generator and op-amp for?
    • What is the extra capacitor between the resistor and ground in the last high-passfilter used for
    I'm sorry if these are obvious questions, but I don't have a lot of feeling for these kind of things yet.

    Thank you in advance

    P.S.: Sorry if this is in the wrong section
  2. dannyf

    Well-Known Member

    Sep 13, 2015
    1. there is no high-pass filter - for stability. the npn/pnp pair is there to amplify the output current;
    2. no positive feedback.
    3. dc blocking + gain setting;
    4. decoupling.
  3. Jony130

    AAC Fanatic!

    Feb 17, 2009
    Q1 is a NPN and Q2 is a PNP and together they form a complementary pair (push-pull "current"amplifier). And they are there to increase opamp output current capability.

    I do not see any positive feedback. Look closer and you will see that R2 and R2 are connected between Vsupply and ground, and together they form a voltage divider.
    When you use BJT as a CE amplifier, you use a voltage divider to bias the active device somewhere in the "linear region".
    In case of single supply op amp you have to do the same think. You need to bias the op amp somewhere in the middle of his "linear region".
    This is a job for R1 and R2 voltage divider.

    C1 and C2 are coupling capacitors. And yes, they form together with resistance a high pass filters. And thous capacitors are needed because the op-amp is supply from a single power supply and we need "separation" between DC and AC (we want to amplify only AC signals).

    R6 and C3 are so called Zobel network or Boucherot cell.
  4. MrAl

    AAC Fanatic!

    Jun 17, 2014

    "High Pass Filter" is probably not the best description of the input and output series capacitors C1 and C2. A better description is "DC blocking capacitors" and that's about it, because they are there mainly to block the DC biasing and pass just the AC part of the signal. The analysis however is like that of a high pass filter because you have to know the low frequency cutoff point, but still they are there mainly to block DC.

    The PNP and NPN transistors operate as voltage followers, which only boost the current of the output so it can drive the low impedance speaker. Their emitters have to be in the feedback loop so that the effect of their emitter base voltage drop can be reduced so that crossover distortion can be minimized. If they where not in the feedback loop the crossover distortion would be tremendous and probably render the amplifier unusable for audio work. When the output of the op amp goes through mid voltage range the opposite transistor has to take over, so the voltage on the base must ramp up quickly from about -0.7v to about +0.7v from nominal (or from positive to negative 0.7v). There is still some crossover distortion however because the op amp is never fast enough to reduce that ramp time to zero due to it's slew rate, but it will be much less than without the feedback.
    Because this shows a square wave input though that raises some questions about why the designer was concerned about crossover distortion, or else maybe they just pulled the design off the web and decided to use it for square waves instead of audio.

    The last resistor R6 and associated capacitor may serve as a multifunctional design device:
    (1) to help smooth out the range of impedance of the speaker over the frequency range, and
    (2) to help reduce the high frequency component of the crossover distortion so that it does not get to the speaker and radiate RF energy back into the front end or into nearby equipment.
    Last edited: Feb 17, 2016