VARIOUS AUDIO CIRCUITS

ECONOMY AMPLIFIER
    When power output, harmonic distortion, frequency response are not the absolute parameters for an amplifier, such as in the case of small personal portable radios, operation of an amplifier in class ‘A’ does have a number of advantages.
    The circuit shown uses -only three transistors, does not require an output transformer, and gives an output of between 100 — 200 mW for a battery supply of only 4.5V.
    RV1 provides volume control and couples into the amplifier through C1. The following three stages are directly coupled.
    Q1 base bias is established by resistors R2 and R5. R1 —Q1 acts as a bias potential divider for Q2 base and similarly R3 — Q2 bias base of Q3.
    R2 and R5 also form part of an overall negative feedback loop
improving frequency response and reducing distortion.
    A compromise between gain and quality results in a choice of values for R6 and C3. C3 is a decoupling capacitor and R6 is adjusted by trial and error. (Minimum value should be 22k.)
 

LOUDSPEAKER MICROPHONE
    A small loudspeaker may be used as a very effective microphone for intercoms etc. if it is coupled via the circuit shown above.
Output is sufficient to drive practically any transistor or valve -audio amplifier.
VOLTAGE CONTROLLED AMPLIFIER

 

 

      When the voltage at the gate of a n-channel MOSFET is varied from 0V — supply volts its resistance varies from about 1 kohm to several tens of megohms. This fact is utilised in the following VCA.
    The inverter is biased into linear operation by the 10m-ohm resistor. When feedback is

applied thegain is set by

 _RF_
 RIN		  
    By allowing a MOSFET to be RIN and RF fixed, with the values shown as the control voltage varies from VDD — VSS the gain of the amplifier varies from cutoff to just over unity.