This document describes the design and implementation of an electret condenser microphone (ECM) pre-amplifier circuit. It details the required components, including transistors, resistors, and capacitors. The circuit uses two transistors in a two-stage design. The first transistor acts as a low-noise amplifier for the microphone signal. Its emitter is decoupled by a capacitor. The second transistor is directly coupled and operates as an emitter follower to provide a flat frequency response from 20Hz to 100KHz with an overall gain of around 100x. Proper soldering technique and cleaning of pads is important during assembly.

1. KUSHAL, 091064
A2 BATCH, ECE
THE ECM PRE AMPLIFIER CIRCUIT
AIM:
To implement and check ECM pre-amplifier circuit.
REQUIRED APPARATUS AND COMPONENTS:
To prepare the Copper track on the PCB
• PCB Board of appropriate size to mount the circuit.
• PCB cutter
• PCB driller
• FeCl3 Solution
• Sand Paper
To mount (Solder) the components on the PCB
• Soldering paste
• Soldering Wire
• Soldering Machine
Extra Apparatus
• DMM
• Ribbon Wire
• Sharp cutter
• Power Supply 6-30 Volts
Components Required
• Transistor BC 549 -- 2
• ECM Mic
• Three 1kΩ, Two 10kΩ and One 22kΩ Resistor
• Two 220µF, Two 10µF and One 100µF capacitor

2. KUSHAL, 091064
A2 BATCH, ECE
THEORY:
ECM:
Electret - An electrical property which describes the capability to retain electrical charges.
Condenser - Another term used for a capacitor: two plates separated by a fixed distance
capable of storing an electric charge.
Microphone - Receiver that converts sound energy to electrical energy.
PRINCIPLE OF WORKING:
An electret condenser microphone (ECM) consists of a very light diaphragm (moving plate)
and back plate (stationary or static plate) and has a permanent charge implanted in an
electret material to provide polarizing voltage. The principle of operation is that sound
waves impinging on the diaphragm cause the capacitance between it and the back plate to
change synchronously, this in turn induces an AC voltage on the back plate.
Circuit Diagram of Pre AMP Circuit:
PRINCIPLE OF WORKING:
The electret condenser microphone (ECM) contains a very sensitive microphone element
and an internal FET preamp, a power supply in the range 2 to 10 volts DC is therefore
necessary. Although the schematic is drawn showing a three terminal ECM, two terminal
ECM may be used.

3. KUSHAL, 091064
A2 BATCH, ECE
The 1k resistor limits the current to the mic. This resistor should be increased to 2k2 if a
supply voltage above 12 Volts DC is used and is not needed if the mic inserted is dynamic.
The first stage amplifier built around Q1 is run at a very low collector current. This factor
contributes to a very high overall signal to noise ratio and low overall noise output. The
emitter resistor of Q1 is decoupled by the 100µF realizing a maximum gain for this stage.
The noise response of the amplifier measured across the 10k load is shown below:
The second phase, built around Q2 is coupled directly, this minimizes the effect of phase
shift(introduced by capacitive and inductive coupling method) and achieve a flat output
response from 20Hz to over 100Khz.
Frequency response measured across 10 kΩ resistor is shown on the next page:

4. KUSHAL, 091064
A2 BATCH, ECE
The emitter voltage of Q2 is also fed back to the base of Q1 via resistive coupling. This also
ensures bias stabilization against temperature effects. Q2 operates in emitter follower
mode, the voltage gain of this stage is less than unity, however, the overall voltage gain of
the preamplifier is about 100X or 20dB as shown in the bode plot above.
Precautions:
1: Do the drilling properly.
2: Iron should be kept in contact with the joint for 2-3 seconds.
3: Put the components at correct place.
4: Clean component lead and copper pad before soldering.