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:
Analogue Essentials Low noise, radio frequency amplifier. Anti-aliasing filter. Power amplification. i.e. The module syllabus.
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.
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.
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.
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
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.
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.
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.
Common-Emitter Limitations It is often not possible to meet a specification using a single amplifier stage High voltage gain AND high current gain can be incompatible Solution: Multi-stage amplifiers using: Differential amplifiers for input Common-emitter amplifier for voltage gain Power amplifier for current gain
Example – An Operational Amplifier + - Differential Amp Voltage Amp Power Amp
Review Topics Focus on review of 1 st and 2 nd year material. In particular Common-Emitter Amplifier Small signal analysis Mutual Conductance Emitter resistance etc.