1. Class A Operating Mode Time I out One device conducts for the whole of the a.c. cycle. Conduction angle = 360 .

2. Class A Output Stage Transistors only conduct current in one direction. 360  conduction angle therefore requires a d.c. bias so that the collector current does not try to go negative. Common circuit design is the Emitter Follower or Common Collector Amplifier.

3. Emitter Follower

4. Emitter Follower Basics As long as the collector current is positive, V BE  0.7 V. V E, therefore equals V B – 0.7 V, i.e. unity voltage gain. Input current will be approximately  times smaller than output current though – i.e. current gain.

5. Emitter Follower Analysis Quiescent Conditions

6. A.C. Response To relate v out to v in, must relate base voltage, v b to v in and v out to v b. vbvb

7. (I) – v out vs. v b vbvb veve

8. (II) - v in vs. v b Potential divider is formed between R S and r in. i IN iBiB i RB

9. Voltage Gain

10. Current Gain

11. Bias Current Two possible ways of approximating the 15 mA current source : ResistorCurrent Mirror

12. Resistor Biasing When the output voltage is at its lowest possible value, the output transistor has just turned off : i.e. maximum output signal has 7.5 V peak amplitude

13. Current Mirror Biasing Again, when the output transistor has just turned off :

14. Resistor vs. Current Mirror Resistor biasing is simpler and cheaper than building a current mirror, however… Maximum output swing is limited using a resistor. The extra resistor appears in parallel with R L, thereby lowering r in. Power dissipation is increased.

15. Microcap simulation #1 EEM3A-1.CIR

16. Summary A class A output stage can be realised by an emitter follower circuit. Voltage gain is approximately unity. Current gain is high. Ideally, a current mirror is required to bias the emitter follower.