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Transistor applications (2)
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- 2. Amplifier Gains • CurrentGain : Ratio of Output current to AI Input current • Voltage Gain : Ratio of Output voltage to AV Input Voltage • Power Gain : Product of Voltage Gain and AP Current Gain (AI . AV)
- 3. Resistors R1, R2and RE form the voltage divider biasing circuit , fixes the Q point. Output Coupling Capacitor with load RL to block any DC component at the output load. Parallel with RE provides low reactance path to the amplified AC signal. Single Stage CE Amplifier Input Coupling Capacitor to block any DC component
- 4. Operation of SingleStage CE amplifier • Consider +ve half cycle of sinusoidal signal as input. • Collector current IC is times IB . • As voltage increases, IC increases. • Increase in IC increases drop across RC . • Since VC = VCC - IC . RC , VC decreases; hence for +ve half cycle, the output is –ve half cycle. • This shows 180o phase shifted waveform at the output of the transistor amplifier. • This is called phase reversal.
- 5. Graphical Representation ofSignals
- 6. Frequency Response • FrequencyResponse curve of the amplifier is a plot of voltage gain against the frequency of input signal. • The amplifier provides the same amplification for few frequency range which is as shown below.
- 7. Bandwidth • The rangeof frequency over which the voltage gain of the amplifier is equal to or greater than 70.7% of the maximum is known as 3db bandwidth of an amplifier. • Bandwidth = f2 – f1 . • Draw a line at 0.707 AVM , the points at which the line cuts the curve gives two frequencies.
- 8. Cascading Amplifier • Connectingoutput of 1st stage as the input of the 2nd stage is called Cascading. Need : • Amplification of single stage is not sufficient. • Output or input impedance is not of required values. • This is done to get required voltage, current and power gain.
- 9. Types of Coupling •To cascade two stages of an amplifier, Coupling circuits are used. Purpose: • Transfers the AC output of one stage to the input of the next stage. • Isolates the DC parameters of two consecutive stages. Types : 1. RC coupling 2. Transformer Coupling 3. Direct Coupling
- 10. Two stage RCcoupled CE Amplifier Coupling Network 1st Stage 2nd Stage
- 11. Two stage RCcoupled CE Amplifier
- 12. Two stage RCcoupled CE Amplifier • Voltage divider biasing circuits are used for each stage for stabilisation. • Capacitors at Emitter provides low reactance path. • Capacitor CC provides DC isolation between the stages.
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- 14. Advantages & Disadvantages •Advantages : i. Most convenient and least expensive multistage amplifier ii. Shows wide frequency response iii. Provides less frequency distortion • Disadvantages i. Provides less voltage gain due to loading effect. ii. Has poor impedance matching • Applications 1. Used in audio amplifiers 2. Used in public address systems
- 15. Transformer coupled CEAmplifier
- 16. Transformer coupled CEAmplifier • Primary winding of transformer is connected in the collector circuit of one stage, as collector load • Secondary winding is connected to the base of the next stage. • Thus the AC signal developed across the primary winding induces the AC signal in the secondary winding which couples this AC signal to the base of the next stage. • The DC isolation is provided by the transformer.
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- 18. Advantages & Disadvantages •Advantages : i. No signal power loss in the collector or base resistor, because of low winding resistance of transformer ii. Provides higher voltage gain than RC coupled iii. Provides excellent resistance matching between stages. • Disadvantages i. The coupling transformer is expensive and bulky, when operated at audio frequencies ii. Poor frequency response iii. At RF, produces reverse frequency distortion iv. Produces ‘hum’ in the circuit
- 19. Applications i. Used forimpedance matching ii. Used as a voltage amplifier and generally used in the last stage of amplifier iii. Used in amplification of radio frequency signals
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- 22. Direct Coupled Amplifier •Amplification of low frequency signals • No coupling element is used. • Advantages: 1. Simple circuit design 2. Less expensive 3. Amplify very low frequency signals 4. Excellent frequency response • Disadvantages: 1. Cannot Amplify high frequency signals 2. Poor thermal stability 3. No DC isolation is provided
- 23. Applications i. Used inelectronic systems that handle signals which change very slowly with time. Some applications are i. Analog Computations ii. Power supply regulators iii. Bioelectric measurements iv. Linear integrated circuits
- 24. Oscillator • Definition: Anoscillator is a circuit that provides a constantly, varying amplified output signal of any desired frequency. • Need : • When waveforms such as sine wave, square, pulse or triangular with specified frequency and amplitude is to be used in electronic systems.
- 25. Range of frequencyof oscillations • Audio Frequency (AF) Oscillators : 20Hz – 20KHz • Radio Frequency (RF) Oscillators : 20KHz – 30Mhz • Very High Frequency (VHF) Oscillators 30 -300MHz • Ultra High frequency (UHF) Oscillators 300MHz – 3GHz • Microwave oscillators 3 – 30GHz.
- 26. Feed Back Oscillator •A part of the output is fed back to the input in proper phase and magnitude such that the effect of feedback increases the input signal. This is called Positive feedback. • Feedback sinusoidal oscillator consists of two essential components: an amplifier and a feedback circuit. • Feedback circuit samples a portion of the output voltage and mixes in phase with the input. • Diagram showing Feedback
- 27. Barkhausen’s criterion • Consideringan amplifier with feedback the gain can be defined as • A’v = Av 1 - . Av • Where, Av = Gain of an amplifier without feedback . Av = loop gain • What happens if . Av =1? • Hence the first condition
- 28. Barkhausen’s criterion • Phaseshift? • The second condition states that phase of the loop gain should be zero or multiples of 360o . • These two conditions are called Barkhausen’s criterion.
- 29. Crystal Oscillator • Apiezoelectric quartz crystal is used as in Crystal Oscillators.
- 30. Crystal oscillator • Piezoelectriceffect : when ac voltage applied across the quartz crystal, it vibrates at the frequency of the applied voltage. • Conversely, if a mechanical force is applied to vibrate a quartz crystal, it generates an a.c. output.
- 31. Electrical Equivalent ofa Crystal • Fig below shows the electrical equivalent circuit of a crystal. • It consists of a series RLC in parallel with capacitance C1 . • When the crystal mounted across the a.c. source is not vibrating, it is equivalent to the capacitor C1 . • When the crystal is vibrating, it acts like a tuned circuit RLC.
- 32. Tank circuit (LC)operation The series resonant frequency of operation is given by fs = 1 2 L . C1 The parallel resonant frequency of operation is given by fp = 1 2 L . C where, C is parallel equivalent of C1 & C2
- 33. Tank circuit (LC)operation Since C2 is very large when compared to C1 and hence C is approximately equal to C1 . Hence fs = fp
- 34. Crystal oscillator circuitResistorsR1, R2 and RE form the voltage divider biasing circuit , fixes the Q point. RFC coil provides the dc bias also couples any ac signal from affecting the output signal Capacitor CE parallel with RE provides low reactance path to the amplified AC signal. Crystal connected as a series element in the feedback path from collector to base Cc with negligible impedance blocks dc between collector and base.
- 35. Applications • Used inapplications where frequency stability is essential. • Used in communication transmitters • Personal Computers • Digital watches and clock