AExE0106 Amplifiers¶
Output-stage purpose¶
An output stage supplies the required load voltage and current while controlling distortion, efficiency, heat, and output impedance. A power amplifier is designed primarily to deliver substantial power to a load; it still has voltage and current relationships, but output power is the design objective.
Class A, B, and AB¶
| Class | Conduction angle per device | Quiescent bias | Main advantage | Main limitation |
|---|---|---|---|---|
| A | \(360^\circ\) | In active region | Best linearity, no crossover gap | High idle dissipation, low efficiency |
| B | \(180^\circ\) | At cutoff | High theoretical efficiency | Crossover distortion near zero |
| AB | \(180^\circ<\theta<360^\circ\) | Slightly above cutoff | Reduced crossover with better efficiency than A | Bias/thermal control needed |
Efficiency limits require topology context:
- series-fed resistive Class A: theoretical maximum \(25\%\);
- ideal transformer-coupled Class A: theoretical maximum \(50\%\);
- ideal sinusoidal Class B push-pull: theoretical maximum \(\pi/4=78.5\%\);
- Class AB lies between Class A and Class B behavior, but has no one universal efficiency percentage.
Class C conducts for less than \(180^\circ\) and normally uses a tuned load to recover a sinusoid. It is efficient for fixed-envelope RF service but is not a linear general-purpose output stage.
Push-pull and crossover distortion¶
A push-pull stage uses two devices so one handles one polarity/half-cycle and the other handles the opposite polarity/half-cycle. It is a power-amplifier output stage.
In an unbiased complementary Class B emitter follower, neither transistor conducts until the input magnitude reaches approximately one base-emitter drop. This creates a dead zone and crossover distortion around zero.
Class AB applies a small forward bias between the transistor bases so both devices conduct slightly near zero crossing. Common bias elements include diode strings and a \(V_{BE}\) multiplier. Because \(V_{BE}\) changes with temperature, thermal tracking and emitter resistors are important to prevent excessive quiescent current and thermal runaway.
Power BJTs¶
Power BJTs are designed for higher current, voltage, and power dissipation than small-signal devices. Selection requires more than checking one current rating:
- maximum \(V_{CE}\), \(I_C\), and power dissipation;
- safe operating area (SOA), including secondary breakdown;
- junction temperature and thermal resistance;
- heat sink and derating;
- saturation voltage and required base drive;
- variable and often lower \(\beta\) at high current.
Instantaneous transistor dissipation is approximately
A device can violate its power/SOA limit before reaching its nominal maximum current.
Transformer-coupled push-pull stages¶
Transformers can:
- split phase for two output devices;
- combine their alternating output currents;
- block DC from the load;
- match a low load impedance to a higher optimum collector load.
Reflected impedance is
Tradeoffs include size, cost, finite bandwidth, copper/core loss, leakage inductance, and possible core saturation. Transformer coupling is historically important and remains useful in selected power/RF applications.
Tuned amplifiers¶
A tuned amplifier uses an LC resonant load and gives high gain over a narrow band around \(f_0\). At resonance, the tank converts device current pulses into a near-sinusoidal voltage. Uses include RF and IF stages, frequency selection, and Class C power amplification.
Do not confuse:
- wideband amplifier: designed for approximately uniform gain over a range;
- tuned amplifier: deliberately narrowband and frequency selective.
Operational amplifiers¶
An op-amp is a very-high-gain differential amplifier:
Ideal assumptions:
| Property | Ideal value |
|---|---|
| Open-loop gain | Infinite |
| Input resistance | Infinite; input currents zero |
| Output resistance | Zero |
| Bandwidth and slew rate | Infinite |
| Input offset and bias current | Zero |
| Common-mode rejection | Infinite |
Real op-amps have finite gain-bandwidth, slew rate, input common-mode range, output current, and output swing. Always check the data sheet in design work.
With negative feedback, linear operation, and no saturation, the ideal rules are:
The second relation is a virtual short, not a physical connection. It is not valid merely because an op-amp symbol is present; negative feedback and linear operation must exist.
Common configurations:
| Circuit | Closed-loop gain | Phase recognition |
|---|---|---|
| Inverting | \(A_v=-R_f/R_{in}\) | \(180^\circ\) inversion |
| Noninverting | \(A_v=1+R_f/R_g\) | No inversion; gain at least 1 |
| Voltage follower | \(A_v\approx1\) | Buffer; high input, low output impedance |
| Comparator | Usually open-loop | Output chooses a saturation state from input polarity |
Amplifier examples¶
- An inverting op-amp with \(R_{in}=10\ \text{k}\Omega\) and \(R_f=50\ \text{k}\Omega\) has \(A_v=-5\).
- A transformer with \(N_p/N_s=5\) reflects an \(8\ \Omega\) load as \(Z_p=5^2(8)=200\ \Omega\).
- A Class B transistor conducts for \(180^\circ\) per cycle, while the pair reconstructs both half-cycles.
Amplifier revision box¶
- Class A: \(360^\circ\); B: \(180^\circ\); AB: between them.
- Class A maximum is \(25\%\) resistive or \(50\%\) transformer-coupled.
- Ideal Class B push-pull maximum is \(78.5\%\).
- Push-pull is a power-amplifier arrangement.
- AB bias reduces the Class B crossover dead zone.
- Tuned amplifier = narrowband LC load, usually RF/IF.
- Op-amp virtual short requires negative feedback and unsaturated operation.