Two-Transistor Radio with Transformers

The next logical progression from the one Transistor radio of project One Transistor Radio with Diode is to this two Transistor receiver.

The antenna and tuning circuitry is the same as project Crystal Set Radio , and the 2SA(1) Transistor amplifier is virtually the same as the one in project One Transistor Radio with Diode. Refer to these projects for the discussion on these items see Radio section.

Also, the experimentation you did on those projects should be tried on this one. The output of the first Transformer feeds the 50K Control. This is necessary to keep from over-driving the final stage and allows a Speaker volume adjustment.

The 10 uF Capacitor couples the audio signal into the base of the 2SA(2) without upsetting the DC voltages required on this stage for proper operation.

The 470uF Capacitor across the Battery is called a "decoupling capacitor". It keeps the two Transistor stages from interfering with each other due to any common Battery impedance which could otherwise cause feedback between these stages.

The final stage is a very common type amplifier. The DC voltage (bias) on this stage is obtained from what has come to be called a "universal bias circuit" because of its universal acceptance in the industry as a very stable circuit.

Collector voltage is supplied through the primary winding of the output Transformer. Base-bias voltage is supplied from a voltage divider made up of the 47K and 22K Resistors. These provide quite a stiff voltage (about 1.6V) on the base of the 2SA(2).

The higher resistance of these two Resistors is called the "base-bias resistor" and the lower value is called the "base-bias divider resistor".

The 100 ohm emitter Resistor, called the "emitter swamping resistor", is used to stabilize the DC bias currents in the circuit by swamping out any tendency of the Transistor to change characteristics.

In this circuit design it was chosen to keep the emitter current near 10mA for all possible Transistor characteristics. Your VOM can be used to measure the voltage drop across this Resistor and then Ohm's law will let you figure the actual emitter current.

Recall I = E/R see Ohm’s Law in Electronic.

If emitter voltage is between 0.8V and 1.2V, the current is within the desired range. A value of about 1V is obtained when the Transistor has operated within the expected characteristics.

The 100uF emitter-bypass Capacitor is used to prevent decreased signal amplification due to degeneration feedback from the voltage across the 100 ohm Resistor. This Capacitor may be disconnected to demonstrate its action. Until you know exactly what to expect, we suggest you don't try changing any Resistor values in the final stage or the Transistor may be damaged. Other projects provide an opportunity for experimentation on this type of circuit.








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