Hearing aid Amplifier
Électronique / Electronic

This is a high gain two Transistor amplifier similar to some of the early models of Transistor hearing aid amplifiers. The Speaker is used as a dynamic microphone. This is a good amplifier on which to use your VOM to help learn how Transistors work. Use the VOM to measure circuit voltages. These voltages may then be used to determine currents and operating characteristics of the circuit. The basic operation is as follows:

The Speaker changes the sound pressure into weak voltages which are increased some by the step-up turns ratio of the Transformer. This voltage is then applied to the B-E input junction of the 2SC Transistor through the 3.3uF Capacitor.

The small 0.01 uF Capacitor has little effect at the audio frequencies, but it stops the ultrasonic oscillations which would otherwise occur because of the high gain of this combined with the long circuit leads.

The amplified voltage at the output of the 2SC Transistor appears across the C-E leads and is coupled into the B-E input junction of the 2SA Transistor through the 0.1 uF Capacitor. This amplified voltage is further amplified by the 2SA and appears across the C-E terminals of this Transistor.

The output voltage is then coupled to the" Earphone through the 100uF Capacitor.

Notice that the above circuit description makes no mention of the Resistors in the circuit. This is because we consider them all as open circuits as far as the audio signal is concerned. The basic purpose of all the resistance in this amplifier is to supply the required DC voltages and currents to the Transistors.

The 1K and 2.2K Resistors are required only to supply the collectors with voltage and current. The 220K and 470K supply base current and voltage.

The type of bias current is the same for both stages and is called "self-current" bias. This is because the collector DC voltage is used to provide the source of current through the base Resistor to the stage with some self stabilizing feedback. The high value of the base resistance (220K and 470K) determines the base-bias current which will flow. Measure the DC voltage across Transistor C-E leads to determine if the Transistors are turned ON by the right amount act as amplifiers.

The voltage (called V CE) should have a value between the OFF value(9V of the Battery) and full ON (0.5V). An electronics technician uses this voltage to verify that the bias is correct for the Transistor to work as amplifier.