How to Get AC Output From DC Input Supply?
How to Get AC Output From DC Input Supply?:
Fig: A push-pull active amplifier circuit & A push-pull switched mode circuit Fig: Switch in amplifier mode operation
Fig: Switched mode (inverter) operation
- In order to have a single control signal for the transistor switches, one transistor is of n-p-n type and the other of p-n-p type and their emitters and bases are shorted as shown in the figures. Both circuits require a symmetrical bipolar dc supply. Collector of n-p-n transistor is connected to positive dc supply ( E) and that of p-n-p transistor is connected to negative dc supply of same magnitude (-E). Load, which has been assumed resistive, is connected between the emitter shorting point and the power supply ground.
- The transistors work in active (amplifier) mode and a sinusoidal control voltage of desired frequency is applied between the base and emitter points. When applied base signal is positive, the p-n-p transistor is reverse biased and the n-p-n transistor conducts the load current.
- Similarly for negative base voltage the p-n-p transistor conducts while n-p-n transistor remains reverse biased. A suitable resistor in series with the base signal will limit the base current and keep it sinusoidal provided the applied (sinusoidal) base signal magnitude is much higher than the base to emitter conduction-voltage drop.
- Under the assumption of constant gain (hfe) of the transistor over its working range, the load current can be seen to follow the applied base signal.
- Accordingly the base magnitudes of current and power are E/R and E2/R respectively. The power loss in switches is a considerable portion of circuit’s input power and hence such circuits are unacceptable for large output power applications.
- The conducting switch remains fully on having negligible on-state voltage drop and the non-conducting switch remains fully off allowing no leakage current through it. The load voltage waveform output by switched-mode circuit of is rectangular with magnitude E when the n-p-n transistor is on and –E when p-n-p transistor is on. In the switched mode (inverter) mode the on and off durations of the two transistors are controlled so that
(i) the resulting rectangular waveform has no dc component
(ii) has a fundamental (sinusoidal) component of desired frequency and magnitude and
(iii) the frequencies of unwanted harmonic voltages are much higher than that of the fundamental component.
- Both amplifier mode and switched mode circuits are capable of producing ac voltages of controllable magnitude and frequency, however, the amplifier circuit is not acceptable in power-electronic applications due to high switch power loss.
- On the other hand, the switched mode circuit generates significant amount of unwanted harmonic voltages along with the desired fundamental frequency voltage. The frequency spectrum of these unwanted harmonics can be shifted towards high frequency by adopting proper switching pattern. These high frequency voltage harmonics can easily be blocked using small size filter and the resulting quality of load voltage can be made acceptable.