The figure below shows the circuit of 20W HiFi amplifier (high fidelity) in Quasi complementary circuit. The final stage is built by the two complementary transistors. T6 and T7 which drive T8 and T9. The final stage connected in front is the front stage T1 and the driver stage T3 transistor 2 located between the bases of the two complementary transistors stabilizes the quiescent current of the final transistor against the fluctuation of the source voltage and ambient temperature when the fluctuation of the source voltage base bias voltage changes and the arm is so the same as the collector current of the stabilizer transistor.
The safety circuit with T4 and T5 prevents overloading of the final transistors in the overload control and short circuits at the output which are not dangerous, and can even return to the quiescent current value, where at the output two fast switching diodes of the BA 145 type are added to limit the overvoltage.
Information:
- BC 178 = driver, to control and amplify the input signal.
- BC 107 = buffer, to meet the input current on Tr BD135 and BD 136.
- BD 135 = buffer, to provide sufficient base current to Tr BD 438.
- BD 136 (2N3702) = buffer, to provide sufficient base current to Tr BD 439.
- BD 438 (BD329) = power amplifier for input signal (+).
- BD 439 (BD138) = power amplifier for input signal (-).
- Point A = component to determine the quiescent current.
- Point B = component that serves to determine the output symmetry.
- Point C = component that functions to suppress high frequency vibrations.
- Point D = as a feedback resistor which aims to widen the amplified audio frequency response.
- Point E = circuit that functions to eliminate cross defects.
- Point F = is the quiescent current at the collector of the power transistor when there is no input signal.
- Point G = component that functions as a fast switch as well as an overvoltage limiter on each power transistor.
OCL Complementary Amplifier
The working principle is when the input signal is ½ wave (+) Tr1 is ON because the base gets bias (+) so that the current flows from the source (+) through Tr1 to LS then to GND. While Tr2 is off because it gets bias (-).
When the input signal is ½ wave (-) Tr23 is ON because the base gets a bias (-) so that the current flows from GND which is more (+) than (-), namely through LS to Tr2 towards the voltage source (-), while Tr1 is off because it gets a bias (+). The above event will continue as long as the input signal still exists.
QUASI Complementary Amplifier
Complementary amplifiers can be strengthened by their output power if we can return 2 final transistors of the same type. The final amplifier stage with a large output power like this requires a driver stage and a front amplifier stage that can provide a large current for the base of the final transistor.
The basic circuit of a quasi-complementary amplifier is shown in the figure below, where T1 and T2, C and LS are connected as a complementary amplifier, each complementary transistor controls one power transistor.
At the ½ wave(+) signal T1 is ON and through the voltage drop on R1, T3 is ON, both transistors flow a large current through LS and charge C.
At signal 1/ wave(-), T2 is ON through the voltage drop on R2, then T4 is ON. Current flows from C through both transistors and LS so that C is discharged. The value of R1 and R2 must be the same, so that both power transistors are controlled in the same form.
Basic Circuit of QUASI Complementary Amplifier