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EEM3A – Analogue Electronics Dr. T. Collins [email protected] http://www.eee.bham.ac.uk/collinst Analogue Electronics ? Who Cares ? Even digital systems usually rely on analogue electronics in some way. E.g. A “digital” radio : R.F. PreAmplifier Power Amplifier D.S.P. Filter Analogue Essentials Low noise, radio frequency amplifier. Anti-aliasing filter. Power amplification. i.e. The course 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. Recap : Common-Emitter Amplifier Quiescent Conditions Assume I B 0 VB 0 VE 0.5 V IE VE 15 0.1 mA RE VC 15 I C RC 15 I E RC 12.5 V Biasing VBE I C I S exp VT 0.12 Collector Current, [mA] 10 8 Slope = gm 0.11 ic 6 0.1 IC vbe 4 0.09 2 VBE 0 0 0.2 0.4 0.6 0.8 Base-Emitter Voltage [V] 1 0.08 0.586 0.590 0.594 Base-Emitter Voltage [V] 0.598 Small Signal Operation • As vin changes, the base-emitter voltage follows, i.e. vin = vbe. • As vbe changes, the collector current follows, ic = gm.vbe. • As ic changes, the voltage across Rc follows (Ohm’s law). • Gain therefore depends on the relationships between vbe & ic and ic & vout. Mutual Conductance, gm Mutual conductance, gm, is simply the slope of the IC-VBE curve. It is not a physical conductance, just the ratio between current and voltage changes. Since the IC-VBE curve is not a straight line, gm changes with bias current. VBE I S VBE I C ic dI C d gm I S exp exp vbe dVBE dVBE VT VT VT VT Voltage Gain ic ic gm vin vbe vout dVout d 15 I C RC RC ic dI C dI C vout vout ic 0.1 RC g m 25k. 100 vin ic vin 25 Equivalent Circuit rin vin iin vin ic vin vin g m iin iRB ib RB RB rin RB || / g m RB || re rout RC Loaded Common-Emitter Amplifier vout g m RC || RL vin i.e. Low load impedance low gain or high gm. But, high gm low re low rin. Power Amplifier Stages Properties : Low voltage gain (usually unity). High current gain. Low output impedance. High input impedance. Example – An Operational Amplifier + - Differential Amp Voltage Amp Power Amp 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. Efficiency / Dissipation The efficiency, h, of an amplifier is the ratio between the power delivered to the load and the total power supplied: PL h PS Power that isn’t delivered to the load will be dissipated by the output device(s) in the form of heat. PD VCE I C PS PL 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. Class A Operating Mode Iout Time One device conducts for the whole of the a.c. cycle. Conduction angle = 360 . Class B Operating Mode Iout Time Two devices conduct for half of the a.c. cycle each. Conduction angle = 180 . Class AB Operating Mode Iout Time Two devices conduct for just over half of the a.c. cycle each. Conduction angle > 180 but << 360 . Class C Operating Mode Iout Time One device conducts a small portion of the a.c. cycle. Conduction angle << 180 . Class D Operating Mode Iout Time Each output device always either fully on or off – theoretically zero power dissipation. 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. 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.