The original motivation for linear amplifiers was to boost the signal on communication lines. The current role is for high quality audio amplification. The basic tool for the linear circuit is the bipolar transistor, the output current is proportional to the base current of the transistor.
Fig. 1: Basic Amplifier
The output voltage is required to lie between the rail voltage Vcc and ground. If the output current is to lie between A and -A amps the current at rest through the transistor must be a least A amps. For amplification of a sinewave, the maximum output power is for a voltage of Vcc peak to peak or an rms value of
The current will have rms value of
thus the power is A.Vcc/4. The average power from the supply is A.Vcc thus the maximum efficiency for this circuit is 25%. To improve the efficiency a push-pull design can be used but this only raises the maximum efficiency to 50%.
Thus an amplifier designed to deliver 1kW to a load must dissipate at least 1kW in heat sinks. The alternative to this is to use the electronics in Switch Mode.
With the linear amplifier the operating point for the switch is to vary along the line AB. When the signal is small the system operates around point C thus half the peak voltage and half peak current will be dissipated in the switch even for zero output.
When a high current is put in the base to the transistor it turns hard ON and will pass current with very little voltage drop across the switch. This gives the operating point B with close to zero power dissipated in the switch. If the base current is fully removed, the transistor turns OFF and operates at A with very little dissipation.
Fig. 2: Voltage Current relation of linear and switching operation.
If now the circuit were operated half the time at A and half at B changing rapidly the average operation will also be at point C but now with ideally zero losses. In practise the ON state will have some losses as no electronic device exists which can carry current with zero voltage drop. This loss is called conduction loss. The OFF state will have some leakage current but this loss effect is usually much smaller than the ON losses and is often ignored.
There is however one loss that can be very significant for switch mode supplies and that is the loss during the change between A and B
Fig .3: Switch mode amplifier
For this discussion the inductor is assumed to be carrying a steady current, the diode is included to provided a current path when the switch is OFF. With the switch OFF the voltage across the switch is Vcc and the current is zero. When the switch is turned ON by supplying base current, the current through the transistor rises but until the switch is carrying the full current the diode is forward biased and the voltage cannot fall. When turning OFF, the voltage rises until the diode if forward biased then the current in the diode can rise and when the current completes its transition then the switching is complete.
Fig. 4: Switching Losses
When this is examined in the voltage current plane the transition can pass through the high V high I region. Thus if the switching is slow the energy loss per switching can be high. For any switch the switching losses will rise with switch frequency and there will be some trade-off of the quality of waveform approximation by high speed switching and the switching losses.
Fig. 5: Voltage - Current switching locus
If we include snubber circuits which use alternate current paths the switch locus can be kept close to the axes and the switch losses significantly reduced.
Fig. 6: Soft Switched amplifier
This circuits contains snubbers to limit the rate of rise of current on turn on and to limit the rate of rise of voltage across the switch on Turn -On. The switch locus now becomes as shown in Fig 7
Fig. 7: Comparison of switching and linear trajectories.
The snubber s mean that the switching losses in the main switch can be much reduced. However the resistive elements mean that the energy is still lost but now from devices easier to heatsink.
The extensions to this circuit which return this energy to the supply or load are known as lossless snubbers. These circuits usually require additional circuit elements for this rise in efficiency.
Copyright © G. Ledwich 1998.
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