1. EEM3A – Analogue Electronics Dr. T. Collins T.Collins@bham.ac.uk http://www.eee.bham.ac.uk/collinst

2. Analogue Electronics ? Who Cares ? D.S.P. Filter R.F. Pre- Amplifier Power Amplifier Even digital systems usually rely on analogue electronics in some way. E.g. A “digital” radio :

3. Analogue Essentials Low noise, radio frequency amplifier. Anti-aliasing filter. Power amplification. i.e. The course syllabus.

4. Power Amplifiers Common-emitter amplifiers and operational amplifiers require high impedance loads. To drive low impedance loads, a power output stage is required. Designs vary in complexity, linearity and efficiency. Power dissipation and thermal effects must be considered.

5. Low Noise and R.F. Amplifiers Pre-amplifier stages are the most prone to noise as the signal level is so low. Careful design minimises interference. Common-emitter amplifiers can have a disappointingly low upper cut-off frequency. Steps can be taken to extend an amplifier’s bandwidth.

6. Active Filters Passive filter designs consist of a ladder of capacitors and inductors. Inductors are bulky, expensive and imperfect components – especially when low values are required. Using operational amplifier designs, inductors can be replaced using a variety of synthesis and simulation techniques.

7. Recap : Common-Emitter Amplifier Quiescent Conditions

8. Biasing 00.20.40.60.81 0 2 4 6 8 10 0.5860.5900.5940.598 0.08 0.09 0.1 0.11 0.12 Collector Current, [mA] Base-Emitter Voltage [V] V BE ICIC v be icic Slope = g m

9. Small Signal Operation As v in changes, the base-emitter voltage follows, i.e. v in = v be. As v be changes, the collector current follows, i c = g m.v be. As i c changes, the voltage across R c follows (Ohm’s law). Gain therefore depends on the relationships between v be & i c and i c & v out.

10. Mutual Conductance, gm Mutual conductance, g m, is simply the slope of the I C -V BE curve. It is not a physical conductance, just the ratio between current and voltage changes. Since the I C -V BE curve is not a straight line, g m changes with bias current.

11. Voltage Gain

12. Equivalent Circuit

13. Loaded Common-Emitter Amplifier i.e. Low load impedance  low gain or high g m. But, high g m  low r e  low r in.

14. Power Amplifier Stages Properties : Low voltage gain (usually unity). High current gain. Low output impedance. High input impedance.

15. Example – An Operational Amplifier + - Differential Amp Voltage Amp Power Amp

16. Power Amplifier Designs Differences between power amplifier designs : Efficiency / Power dissipation. Complexity / Cost. Linearity / Distortion. Power amplifier designs are usually classified according to their conduction angle.

17. Efficiency / Dissipation The efficiency, , of an amplifier is the ratio between the power delivered to the load and the total power supplied: Power that isn’t delivered to the load will be dissipated by the output device(s) in the form of heat.

18. Conduction Angle The conduction angle gives the proportion of an a.c. cycle which the output devices conduct for. E.g. On all the time  360  On half the time  180  etc.

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

20. Class B Operating Mode Time I out Two devices conduct for half of the a.c. cycle each. Conduction angle = 180 .

21. Class AB Operating Mode Time I out Two devices conduct for just over half of the a.c. cycle each. Conduction angle > 180  but << 360 .

22. Class C Operating Mode Time I out One device conducts a small portion of the a.c. cycle. Conduction angle << 180 .

23. Class D Operating Mode Time I out Each output device always either fully on or off – theoretically zero power dissipation.

24. Differences Between Classes Class A : Linear operation, very inefficient. Class B : High efficiency, non-linear response. Class AB : Good efficiency and linearity, more complex than classes A or B though. Class C : Very high efficiency but requires narrow band load. Class D : Very high efficiency but requires low pass filter on load.

25. Summary Multi-stage amplifiers generally consist of a voltage gain stage and a current gain (or power amplifier) stage. Several operating modes for power amplifiers can be designed. Major differences between modes are efficiency, complexity and linearity.