Common Emitter AmplifierOperation, performance and circuit analysis

This report is written by Mehrzad Fereidooni (000459259)

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Table of Content:

1 _) . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction and background

2 _) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical measurements

2-1 . . . . . . . experimental circuit

2.2 . . . . . . Equipment and components

2.3 . . . . . . . Experimental Procedure

2.4 . . . . . . Experimental Results

2.4.1 . . . . DC conditions of single Amplifier

2.4.2 . . . . Frequency Response of single amplifier with single Cc

3_) . . . . . . . . . . . . . . . . . . . . . . .. . . . .. . . Theoretical Analysis

3.1 DC Conditions of Single Amplifier

3.2 The Midband Gain of Single Amplifier

3.3 The High Cut-off Frequency of Single Amplifier

3.4 The Cut-off Frequency with Single Cc

4_) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PSICE Simulation

4.1 DC conditions of single amplifier

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4.2 Frequency response of single amplifier with double Cc

5_) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion

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1_) Introduction and background:

The purpose of this report is to study the performance of common emitter amplifier, this include the practical measurement shown in tables below and all the theoretical and Pspice simulation.

The common emitter configuration is one of most commonly used BJT amplifier circuits. This type of circuit produces a curve known as the output characteristic curve, this curve is the product of the collector current the (Ic), to the output voltage the (Vce); for a set of different value of base current the (Ib), basically the input current and the output voltage are the independent variables where as the input voltage and the output current are depended variables.

As for all the transistor circuits we need an AC condition to bias the transistor this is very important to achieve the right operating point and reduce any distortion at the output signal.

In this lab session Frequency response of the BJT common emitter amplifier is studied by measuring the overall voltage gain versus frequency. Those measurements were used to analyse behaviour of common emitter amplifier and figure out the properties like midband, midband gain, low-frequency band, low cut off frequency, high-frequency band, high cut off frequency and finally the Bandwidth. The main aim of this assignment is to compare those practically measured properties with the theoretical demonstration and simulation results to get insight about the frequency performance of a common emitter amplifier.

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2.) Practical Measurements

2.1 Experimental Circuit

Once the Q point has been established by biasing the circuit, an input voltage can be applied through coupling capacitors C1 and C2; they isolate the dc signal of the biasing circuit from the input signal.

Figure 1.Schematic diagram of the circuit.

2.2 Equipment and components.

1)Regulated DC supply.2)Function generator.3)Oscilloscope.4)

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Breadboard.5)BC 337 NPN transistors. X 26)Resistors. (56kΩ, 22kΩ, 180Ω, 4.7kΩ, 3.3kΩ, 10kΩ and 1kΩ)7)Capacitors. (330nF, 470pF (×2), 220µF and 10µF)

2.3 Experimental Procedure.

1)Left hand side of the dotted line AB of the circuit shown in figure 1 was built without the capacitors. DC voltages at the emitter, the base, the collector were measured.2)C1,CE,Cc capacitors were added and the CL+RL load were added to the collector of Q1.20mV peak to peak sine wave was applied to the input and gain was measured at the frequency of 5KHz3)Frequency response of the amplifier was measured varying the frequency from 10Hz to 100KHz.Measures values were plotted to find low and high cut-off frequencies.4)

Capacitor value of Cc was doubled adding another capacitor with same value in parallel and step 3 was repeated to take another set of measurements.5)

Q2 was added between CL and RL and the collector of Q1 as shown in the right hand side of the doted line AB of the circuit in Figure 1.DC values of the base and emitter voltages without the signal.

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6)Midband gain was Re-measures plotting the results and high and low cut-off frequencies were checked using the plot.

2.4 Experimental Results.

2.4.0 DC conditions of single Amplifier.

We all know and assume that Vbe should be 0.7 that is because the transistor at that point is in active mode. In cut off mode (Ie=0) the current enters the collector terminal and leaves the base terminal, the voltage drop across the base and collector are very low in regions of millvolts with respect to change in temperature.

4.12V 0.6V

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VB1 VE1 VC1

3.94V 3.35V 9.89V

Where and

9.37V 0.6V 9.37V 8.83V 15V

2.4.1Cascade Amplifier Dc Conditions

2.4.2 Frequency Response of single amplifier with single Cc.

Figure 2.Frequency response with single Cc

a) From the figure lower cut-off frequency=102.146 ≈139hzb) From the figure High cut-off frequency=104.586≈40kHz

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The overall gain equals to the product of the gain of the individual stages, the gain bandwidth product of the cascade is increased in comparison to that of a single capacitor single stage. For a specified value of Fh, the gain of cascading amplifier is greater than that achieved by a single stage having the same Fh.

2.4.2 Frequency Response of single amplifier with

double Cc.Figure 3.Frequency Response with Double Cc

a) From the figure lower cut-off frequency =101.985≈100Hzb) From the figure High cut-off frequency=104.294≈20kHz

3. Theoretical Analysis

The high frequency response of the amplifier is determined by a single time constant at the first part with a single capacitor, but a multi stage consist of at least two capacitors. Also including a resistance in the signal pass between the ground and emitter can led to significant changes in amplifier characteristic.

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3.1 DC Conditions of Single Amplifier

Determining I c, rπ1 and gm1.

Figure 4.DC conditions 1

Figure 5.DC conditions 2

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IC1=1.045mA

3.2 The Midband Gain of Single Amplifier

Small signal equivalent model at midband frequency is shown in figure 6.

In our analysis we assume that the amplifier is working in midband range, the signal frequency is sufficiently high so C1 and C2 are short circuited

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Figure 6. Small signal equivalent model

From the figure 8 Midband gain Am1 can be found as follow:

Midband Gain Am= 30 dB

3.3 The High Cut-off Frequency of Single Amplifier

3.3.1 The Cut-off Frequency with Single Cc

In the High Frequency band gain falls off as a result of Cπ and Cµ .The High frequency equivalent circuit model is shown in figure 7.

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Figure 7.High Frequency equivalent model

From millers Theorem:

The bandwidth of the common-emitter amplifier tends to be low due to high capacitance resulting from the Miller effect . The parasitic base-collector capacitance appears like a larger parasitic capacitor from the base to ground [1]. This large capacitor greatly decreases the bandwidth of the amplifier as it makes the time constant of the parasitic input RC filter where is the output impedance of the signal source connected to the ideal base.

Figure 8.Simplified circuit 1

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Figure 9.Simplified circuit 2

If effective source resistance is considered as zero so “ RB’//RS ” becomes zero .So we can consider RB’(=R1//R2) is not present. Short circuiting the source effective resistance (Rin) can be calculated as follows.

Rin=RB//Rπ

From the values we can see Cc is much larger than Cµ.So we can neglect Cµ and replace “(Cµ+ Cc)” with Cc. But in the other hand effect of Cπ is neglect able compare to the magnitude of “Cc(1+gmRL

’)” So Cin becomes as follows.

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Now the whole circuit can be simplified in to a simple RC circuit as shown in Fig 10.Using the RC circuit High cut-off frequency can be determined as follows:

3.3.2 The Cut-off Frequency with Double Cc

The increaser of Cc can be considered as a increaser of the internal capacitance. Earlier from the equation 5 it was apparent effect of Cπ and Cµ can be neglected as Cc is much bigger than both of them. So as Cc was doubled new Cin can be calculated multiplying the equation 7 by 2( (7)x2) .

Rin Is equal the value calculated in equation 6

So High cut-off frequency can be calculated as follows

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When Bandwidth it taken Lower cut off frequency can be ignores as high cut-off frequency is much larger than low cut-off frequency. So BW was almost decreased by the factor of ½ by doubling the Cc.

4. PSICE Simulation.

4.1 DC conditions of single amplifier.

Psice file*Resistors in CircuitRB 2 4 180RC 5 1 4k7RE 3 0 3k3R1 4 0 22kR2 5 4 56k*Diode in CircuitQ1 1 2 3 QBC337-25.MODEL QBC337-25 NPN( + IS = 4.13E-14 + NF = 0.9822 + ISE = 3.534E-15 + NE = 1.35 + BF = 292.4 + IKF = 0.9 + VAF = 145.7 + NR = 0.982 + ISC = 1.957E-13 + NC = 1.3 + BR = 23.68 + IKR = 0.1 + VAR = 20 + RB = 60

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+ IRB = 0.0002 + RBM = 8 + RE = 0.1129 + RC = 0.25 + XTB = 0 + EG = 1.11 + XTI = 3 + CJE = 3.799E-11 + VJE = 0.6752 + MJE = 0.3488 + TF = 5.4E-10 + XTF = 4 + VTF = 4.448 + ITF = 0.665 + PTF = 90 + CJC = 1.355E-11 + VJC = 0.3523 + MJC = 0.3831 + XCJC = 0.455 + TR = 3E-08 + CJS = 0 + VJS = 0.75 + MJS = 0.333 + FC = 0.643)**Vcc 5 0 15.OP.END

OutputNODE VOLTAGE NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE

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( 1) 10.2790 ( 2) 4.1647 ( 3) 3.5529 ( 4) 4.1654

( 5) 15.0000

S

4.2 Frequency response with single Cc

Pspice file

*Resistors in CircuitRB 4 2 180RC 5 1 4.7kRE 3 0 3.3kR2 4 0 22kR1 5 4 56kRL 7 0 1k*Capacitors in CircuitCc 2 1 470pFCL 1 7 10uFCE 3 0 220uFC1 8 4 330nF*Diode in CircuitQ1 1 2 3 QBC337-25.MODEL QBC337-25 NPN( + IS = 4.13E-14

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+ NF = 0.9822 + ISE = 3.534E-15 + NE = 1.35 + BF = 292.4 + IKF = 0.9 + VAF = 145.7 + NR = 0.982 + ISC = 1.957E-13 + NC = 1.3 + BR = 23.68 + IKR = 0.1 + VAR = 20 + RB = 60 + IRB = 0.0002 + RBM = 8 + RE = 0.1129 + RC = 0.25 + XTB = 0 + EG = 1.11 + XTI = 3 + CJE = 3.799E-11 + VJE = 0.6752 + MJE = 0.3488 + TF = 5.4E-10 + XTF = 4 + VTF = 4.448 + ITF = 0.665 + PTF = 90 + CJC = 1.355E-11 + VJC = 0.3523 + MJC = 0.3831 + XCJC = 0.455 + TR = 3E-08

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+ CJS = 0 + VJS = 0.75 + MJS = 0.333 + FC = 0.643)*Vcc 5 0 15Vin 8 0 AC 20m*.AC DEC 100 10 1Meg.PROBE.END

Figure 10- single amplifier frequency response

4.3 Frequency response of single amplifier with double Cc

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PSPICE file

*Resistors in CircuitRB 4 2 180RC 5 1 4.7kRE 3 0 3.3kR2 4 0 22kR1 5 4 56kRL 7 0 1k*Capacitors in CircuitCc 2 1 940pFCL 1 7 10uFCE 3 0 220uFC1 8 4 330nF*Diode in CircuitQ1 1 2 3 QBC337-25.MODEL QBC337-25 NPN( + IS = 4.13E-14 + NF = 0.9822 + ISE = 3.534E-15 + NE = 1.35 + BF = 292.4 + IKF = 0.9 + VAF = 145.7 + NR = 0.982 + ISC = 1.957E-13 + NC = 1.3 + BR = 23.68 + IKR = 0.1 + VAR = 20 + RB = 60 + IRB = 0.0002 + RBM = 8 + RE = 0.1129

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+ RC = 0.25 + XTB = 0 + EG = 1.11 + XTI = 3 + CJE = 3.799E-11 + VJE = 0.6752 + MJE = 0.3488 + TF = 5.4E-10 + XTF = 4 + VTF = 4.448 + ITF = 0.665 + PTF = 90 + CJC = 1.355E-11 + VJC = 0.3523 + MJC = 0.3831 + XCJC = 0.455 + TR = 3E-08 + CJS = 0 + VJS = 0.75 + MJS = 0.333 + FC = 0.643)*Vcc 5 0 15Vin 8 0 AC 20m*.AC DEC 100 10 1Meg.PROBE.END

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Figure 11.Frequency response of single amplifier with double Cc

Table-1) frequency response with double capacitors:

double capacitor

frequency Vin Vout gain gain

Hz mV mVVin/Vout db

50Hz 20mV 0.9v 22.22 26.93db100Hz 20mV 1.8v 11.11 20.91db200Hz 20mV 2v 10 20db400Hz 20mV 2v 10 20db500Hz 20mV 2v 10 20db600Hz 20mV 2v 10 20db800Hz 20mV 2v 10 20db1KHz 20mV 2v 10 20db2KHz 20mV 2v 10 20db4KHz 20mV 1.75v 11.42 21.15db5KHz 20mV 1.5v 13.33 22.49db6KHz 20mV 1.5v 13.33 22.49db8KH 20mV 1.25v 16 24.08db10KHz 20mV 1.1v 18.18 25.19db11KHz 20mV 1v 20 26.02db12KHz 20mV 0.9v 22.22 26.93db13KHz 20mV 0.9v 22.22 26.93db

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14KHz 20mV 0.8v 25 27.95db15KHz 20mV 0.8v 25 27.95db16KHz 20mV 0.7v 28.57 29.11db17KHz 20mV 0.7v 28.57 29.11db18KHz 20mV 0.7v 28.57 29.11db19KHz 20mV 0.6v 33.33 30.45db20KHz 20mV40KHz 20mV60KHz 20mV80KHz 20mV100KHz 20mV200KHz 20mV400KHz 20mV600KHz 20mV800KHz 20mV1MHz 20mV

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Table_ 2) frequency response of single capacitor measured in the lab

single capacitor

frequency Vin Vout gain gain

Hz mV mVVin/Vout db

50Hz 20mV 1.5mV 13.33 22.496db100Hz 20mV 1.5mV 13.33 22.496db200Hz 20mV 1.5mV 13.33 22.496db400Hz 20mV 1.5mV 13.33 22.496db500Hz 20mV 1.5mV 13.33 22.496db600Hz 20mV 1.5mV 13.33 22.496db800Hz 20mV 1.5mV 13.33 22.496db1KHz 20mV 1.5mV 13.33 22.496db2KHz 20mV 2.0mV 10 20db4KHz 20mV 2.0mV 10 20db5KHz 20mV 2.0mV 10 20db6KHz 20mV 2.0mV 10 20db8KH 20mV 1.75mV 11.42 21.15db10KHz 20mV 1.5mV 13.33 22.49db

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11KHz 20mV 1.625mV 12.3 21.79db12KHz 20mV 1.5mV 13.33 22.49db13KHz 20mV 1.5mV 13.33 22.49db14KHz 20mV 1.45mV 13.79 22.79db15KHz 20mV 1.375mV 14.54 23.25db16KHz 20mV 1.35mV 14.81 23.41db17KHz 20mV 1.25mV 16 24.08db18KHz 20mV 1.15mV 17.39 24.80db19KHz 20mV 1.15mV 17.39 24.80db20KHz 20mV 1mV 20 26.02db40KHz 20mV 0.75mV 26.66 28.51db60KHz 20mV 0.5mV 40 32.04db80KHz 20mV 0.4mv 50 33.97db100KHz 20mV 0.3mv 66.66 36.47db200KHz 20mV 0.275mV 72.72 37.23db400KHz 20mV 0.2mV 100 40db600KHz 20mV 0.2mV 100 40db800KHz 20mV 0.2mV 100 40db1MHz 20mV 0.2mV 100 40db

As we can see in this graph for a constant and continuous value of voltage input at 20mV the voltage output varies.

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As the frequency increses the voltage output increases up to a certeian point then starts to decline.

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As we increase the frequency we notice that the gain also increases.

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Conclusion:

From what we have learned and experimented we could say that only the common-emitter stage is capable of both voltage gain and current gain greater than unity, this make common emitter model very versatile model. The voltage gain of a common emitter stage depends on the value of β. Another important factor is the resistor, the resistance in the emitter enables the amplifier to handle large input signal without incurring nonlinear distortion. This is because only a fraction of the input signal appears between the base and the emitter.

The common emitter configuration is the best suited in realizing the bulk of gain required in an amplifier, depending on the magnitude of gain required, either in a single stage or a cascade of two or more stages. Because of the high input resistance of CE the high frequency response of this model is grey. In situation where R is relatively large and C is relatively small miller’s theorem can be used to obtain a quick and accurate model of higher cut off frequency. In common emitter the low-frequency power gain obtained with a resistive load is much higher than any other configuration.

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Reference:

Dr Ruiheng Wu analogue electronics lectures, university of greenwich

http://en.wikipedia.org/wiki/Cascode

http://www.electronics-tutorials.ws/amplifier/amp_2.html

Analysis and design of analogue integrated circuits/ ISBN 978-0-39877-7

Microelectronics by Jacob millman ISBN 0-07-100596-x

Analogue circuit design by Jim Williams ISBN 0-7506-9166-2

Microelectronic circuit by Rashid ISBN-10:0-534-95174-0

Analogue electronic circuit design by Jan Davidse

ISBN 0-13-035346-9

Microelectronic circuit Sedra /Smith ISBN 0-19-514252-7

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