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2. The Practical, Non-Ideal Amplifier Part I The Real Amplifier: power supply rail Ri RO A RO 0 RS signal source Vi Ri VO VL RL load Vs Properties: Finite Input Resistance, Ri The input resistance is finite and not unlimited. This means that the source must deliver a current (or power) to the input of the amplifier. The input signal level present at the input to the amplifier depends on the relative values of the source resistance and amplifier input resistance. There can be attenuation of the signal generated by the source on reaching the input of the amplifier. Power may be dissipated by both the source resistance and the amplifier input resistance. 1 Finite Ouput Resistance, RO 0 The ouput resistance of the amplifier is not zero but is finite There is no limiting factor on the output current available from the amplifier. The signal present at the load and the power delivered to the load only depends on the load resistance. The current or power which can be delivered to the load is limited by the output resistance. Considerable power can be dissipated in the output resistance of the amplifier and this can generate heat. For high power applications means of cooling the amplifier may be required. Limited Bandwidth, BW Bandwidth is not infinite but limited. There is a maximum frequnecy which is passed by the amplifier with full gain. There is a limit to the rate of change in the signal which the amplifier can handle. Frequency components outside of the bandwidth of the amplifier are attenuated and this can lead to distortion f the signal. AV fMAX 2 Gain Variation In simple amplifers manufacturing variations in the semiconductor fabrication process give rise to large fluctuations in the amplifying properties of the devices used to construct the amplifier. This gives rise to varition in the amplifer gain, which can only be overcome by more advanced design techniques such as the use of feedback. 3 4 Practical Amplifier Limitations Input Attenuation In practice, because of the finite input resistance of the amplifier, not all of the source signal ‘potentially available’ is transferred to the input of the amplifier. RS signal source VS VS Under open-circuit conditions (i.e. no load present) the full signal voltage VS appears at the terminals of the source as no input current flows. Ii A RS signal source VS Vi Ri When the source is connected to the input of the amplifier, the voltage appearing at the terminals, Vi, is less than the open-circuit voltage, VS. 5 Using Ohm’s Law Ii VS RS Ri Then the potential developed across the input resistor, Ri is: Vi Ii R i VS Ri Ri VS RS Ri RS Ri The input attenuation factor is defined as the fraction of the open-circuit or ‘potentially available’ source signal which is actually transferred to the inut of the amplifier. The Attenuation Factor, i is the ratio of the input voltage, Vi to the source voltage VS. Then αi Vi Ri VS R S R i Rarely do we want this attenuation factor to exceed 0.8 to 0.9 or 80% to 90% This requires R i R S Ideally Ri or 6 RS 0 Case Study 1: A transducer has a source resistance of 10kΩ. The input resistance of the associated amplifier is 100kΩ. If the transducer generates an unloaded signal voltage of 1V rms, determine the signal present at the amplifier input and the degree of attenuation present. Ii A RS signal source VS Vi Ri VS Ri Vi Ii R i Ri VS RS Ri RS Ri 105 1 Vi I i R i 4 x1 0.9V 5 1.1 10 10 Vi Ri 1 αi 0.9 VS R S R i 1.1 7 Case Study 2: A moving coil pick-up, having an internal resistance of 50kΩ, produces an unloaded signal level of 8mVrms. An audio preamplifier stage must produce an open-circuit output signal level of at least 0.5V rms with a voltage gain of 100. Determine the minimum input resistance required in the amplifier and the current which the pick-up must deliver to the amplifier. Ii A RS signal source VS VO AV 100 Vi ; Vi Ri VO .5V ; VS 8mVrms Then: VO 500mV Vi 5mVrms AV 100 Vi Ri VS RS Ri 5mV 8 Ri x8mV RS Ri Ri 5 8 RS Ri Then: 5R S R i 8R i 5R S 8R i 5R i 5R S 3R i 5 R i MIN R S 3 R i MIN 5x50kΩ 250kΩ 83.3kΩ 3 3 The input current which the pick-up must deliver to the amplifier input is: VS 8mV 8x10 3 Ii R S R i 50kΩ 83.3kΩ 133.3x10 3 VS 8mV 8x10 3 Ii R S R i 50kΩ 83.3kΩ 133.3x10 3 8x10 6 Ii 0.06μ. 133.3 or 60nA Note: the low level of current associated with this relatively sensitive transducer. 9