The essential features of this type of amplifier are that both transistors are cut off under quiescent conditions, that neither transistor conducts until its input drive signal exceeds its base-emitter 'knee' voltage, and that one transistor is driven on when the other is driven off, and vice versa. In this particular design, Q1 and Q2 are wired in the common-emitter mode and drive the loudspeaker via push-pull transformer T2, and the anti-phase input drive is obtained via phase-splitting transformer T1. General-purpose high-gain low-power audio amplifier.Ī basic class-B amplifier consists of a pair of transistors, driven in anti-phase but driving a common output load, as shown in Figure 3(a). Input pot RV1 acts as the circuit's volume control.įIGURE 2. The bias is thus stabilized by DC negative feedback. Q1's base bias is derived (via R2) from Q2's emitter, which is decoupled via C3 and thus 'follows' the mean collector voltage of Q1. Q1 and Q2 are wired as direct-coupled common-emitter amplifiers, and give an overall voltage gain of about 80 dB. Basic circuit (a) and transfer characteristics (b) of a class-A amplifier.įigure 2 shows an example of a low-power (up to a few dozen milliwatts) high-gain general-purpose class-A amplifier that draws a quiescent current of about 20 mA and is suitable for driving a medium impedance (greater than 65Ω) loudspeaker or headset. The circuit consumes a high quiescent current, and is relatively inefficient 'efficiency' is the ratio of AC power feeding into the load, compared with the DC power consumed by the circuit, and at maximum output power is typically about 40%, falling to 4% at one tenth of maximum output, etc.įIGURE 1.
This action can be achieved by operating the transistor(s) in either of two basic modes, known as 'class-A' or 'class-B.'įigure 1(a) shows a basic class-A audio amplifier circuit Q1 is a common-emitter amplifier with a loudspeaker collector load, and is so biased that its collector current has a quiescent value halfway between the desired maximum and minimum swings of output current, as shown in Figure 1(b), so that maximal low-distortion output signal swings can be obtained. POWER AMPLIFIER BASICSĪ transistor power amplifier's job is that of converting a medium-level medium-impedance AC input signal into a high-level low-impedance state suitable for driving a low-impedance external load. The installment concludes by presenting a practical 'scratch and rumble' filter circuit, which can be used to eliminate these unwanted sounds when playing old-fashioned records/discs through any type of audio power amplifier system. This month we describe the operating principles of various circuits of this type and present a selection of practical audio power amplifier circuit designs. One of the most popular applications of transistors is in audio power amplifiers.
0 kommentar(er)
