While flowing up the discussion on bestengineeringprojects.com, one of our reader ask about dynamic microphone pre-amplifier. The circuit of dynamic microphone is bit different than other microphone pre-amplifier circuit because it consists two stage amplifier section. Thus, before going through circuit description we would like to make clear about dynamic microphone.
What is Dynamic Microphone?
A dynamic microphone is a transducer which convert sound signal into electrical signal and its work on electromagnetic induction principal because it contains moving coil. This microphone is also called moving coil microphone. It consists of conductor in the form of coil which placed between two strong permanent magnet poles. The duralumin metal diphagram is attached to the moving coil made from aluminum ribbon. Dynamic microphone construction is shown in figure 1.
Circuit Description of Dynamic Microphone Pre-amplifier
The circuit of dynamic microphone amplifier is shown in figure 2. The circuit is basically a three-stage discrete amplifier. This circuit has in built gain control. We had divided the entire circuit into two different section for better description.
Input Stage: For input stage, the consideration is not the maximum voltage gain but the impedance matching of the source with the input impedance of the input stage. Some driving sources may require input circuit to be an almost open circuit while others need an almost short circuit. Hence, we may be required to use CC (Common Collector) or CB (Common Base) configuration for the input stage for proper impedance match at the voltage or current gain. In certain other cases, choice of configuration for the input stage is the minimization of noise and maximization of signal/noise power ratio. Thus, we had chosen common base transistor amplifier.
Transistor T2 and T3 is direct coupled amplifier which is also referred as multistage amplifier. Choice of transistor configuration to be used for intermediate stage in a multistage amplifier depends on the maximum voltage gain provided by the configuration. Hence, CC configuration can not be used since a stage of CC amplifier gives voltage gain less than unity. A single stage CB amplifier, no doubt, gives voltage gain more than unity but intermediate stage can not use even CB configuration since the overall voltage gain of multistage amplifier using CB configuration is low, almost equal to the voltage gain of the last stage alone. This becomes evident from the following discussion.
The voltage gain AVk of any stage, say Kth stage is given by,
But RLk = Rck || Ri(k+1)
Hence RLk < Ri(k+1)
Assuming the stage to be identical i.e. assuming that Ri(k+1) = Rik, we conclude that RLk < Rik. Further the maximum value of current gain AIK in CB configuration is hfb which is slightly less than unity. Hence as per above equation, AVk for any CB stage except the last stage is less than unity. Hence, we conclude that even CB configuration is not suitable for use in intermediate stage.
CE stage are popular used for intermediate stage since hfe for a CE stage is much greater than unity.
The over all gain of this circuit is +39dB which is approx. 90 time of input signal. The gain of the circuit can be controlled using variable resistor VR1. This circuit can be suitable for 30Hz to 100kHz signal.
Note: The circuit posted here is suitable for 200-ohm to 600-ohm dynamic microphone.