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Transistors Fundamentals What transistors do How to analyse transistor circuits Small and large signals Common-Emitter Amplifier Review of analysis and design

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The Bipolar Junction Transistor BJT is a current amplifier The collector current is controlled by a much smaller base current The sum of the collector and base currents flow into or out of the emitter Base-emitter junction looks a lot like a PN junction diode

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Operating Regions - Cut Off If the base current is zero, the collector current is also zero It doesn’t matter how big the collector- emitter voltage, V CE, is i.e. collector-emitter junction looks like an open circuit In this state, the transistor is in the cut-off region

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Operating Regions - Active Base current flows and controls the larger collector current Collector current is proportional to the base current Transistor is in the active region Operation can be summarised by two equations: V BE ICIC IBIB

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Operating Regions - Saturation Collector current rises in proportion to the base current As collector current rises, resistor voltage rises and collector-emitter voltage falls When V CE 0, it can’t go any lower and the collector current cannot get any higher The transistor is saturated Collector-emitter junction looks like a short circuit

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Amplification BJT amplifiers work by controlling the collector current by the base-emitter voltage This is only possible in the active region Cut-off and saturation regions correspond to the transistor turning fully ‘off’ or ‘on’ like a switch In the active region, the transistor is only partly ‘on’ and the current can be controlled

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Small Signals We want circuits with a linear response but real transistors aren’t linear Voltage Current = v = i Small variations (i.e. signals!) are denoted by lower case If the range of voltages/currents is kept small, response is approximately linear II VV V I Average (or quiescent) levels are denoted by capital letters

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Small Signal Collector Current

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Mutual Conductance I C and V BE are exponentially related i C and v BE, on the other hand, are approximately linearly related The constant of proportionality, g m, is known as the mutual conductance It isn’t a real conductance, but it is the ratio between a current and a voltage

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Estimating g m The small signal behaviour is estimated by a tangent to the exponential I C -V BE curve g m is, therefore, simply the gradient of the curve

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Amplification V BE ICIC Assume that the transistor is biased in the active region somehow… Collector voltage, V C, is related to I C by Ohm’s law Small signal ratio between collector voltage and collector current is: So: RCRC VCVC

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Simple Common-Emitter Amplifier I B provides a d.c. base current to bias the transistor in the active region C IN couples the input voltage, removing the d.c. base bias voltage C IN is a short circuit to a.c. signals… …but an open circuit to the d.c. bias current v BE is, therefore, equal to v IN

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Analysis

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Biasing Gain is proportional to g m which is, in turn, proportional to I C In this circuit, Unfortunately, has a very wide tolerance The gain is, therefore, not predictable

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Reliable Biasing Collector current is set accurately regardless of C E ensures that the whole of the a.c. input voltage is still dropped across V BE R B provides the d.c. base bias current Usually, the current source is approximated by a resistor

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Practical Amplifier To analyse the circuit: Determine quiescent conditions Calculate mutual conductance Calculate small signal performance Voltage Gain Input Impedance Output Impedance Cut-off frequency

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The Story so Far Small signal analysis is used to simplify calculations by ‘linearising’ the non-linear response of the transistor Using mutual conductance, gain calculations are now only a couple of lines of equations Careful choice of the biasing network leads to reliable performance Next time – practical amplifier calculations, input & output impedances and capacitor calculations

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