Transistor Configuration 1
Objectives To review BJT biasing circuit. To study BJT amplifier circuit To understand the BJT configuration. To analyse single-stage BJT amplifier circuits. To study the differential amplifier. To design amplifier circuits. 2
Transistor Biasing Circuits Voltage Divider Bias Emitter Bias Base Bias Emitter-feedback Bias Collector-feedback Bias 3
Voltage Divider Bias Voltage-divider bias is the most widely used type of bias circuit. Only one power supply is needed and voltage-divider bias is more stable( independent) than other bias types. For this reason it will be the primary focus for study. 4
In the case where base to ground resistance(input resistance) is low enough to consider, we can determine it by the simplified equation R IN(base) = DC R E We can view the voltage at point A of the circuit in two ways, with or without the input resistance(point A to ground) considered. 5
Thevinizing the bias circuit 6
Stiff voltage divider Stiff voltage divider: (No loading effect) 1. I B is much smaller than I 2 2. R IN(BASE) = DC R E >10R 2 V B = (R 2 /(R 1 + R 2 ))V CC I E I C Non-Stiff voltage divider: (with loading effect) V B R 1 R 2 DCR ( R 2 DC E R E V ) CC 7
We now take the known base voltage and subtract V BE to find out what is dropped across R E. Knowing the voltage across R E we can apply Ohm s law to determine the current in the collector-emitter side of the circuit. Remember the current in the base-emitter circuit is much smaller, so much in fact we can for all practical purposes we say that I E approximately equals I C. I E I C 8
Example Determine the dc input resistance looking at the base of the transistor if DC = 125. 9
Example Determine V CE and I C in the stiff voltage divider biased transistor circuit if DC = 100. 10
Example Find I C and V EC for the pnp transistor circuit as given. 11
Example Find I C and V CE for the pnp transistor circuit as given. Assume R 1 =68 kω, R2= 47 kω, R C = 1.8 kω,r E = 2.2 kω, V CC = -6 V and DC = 75. 12
Emitter Bias This type of circuit is independent of making it as stable as the voltage-divider type. The drawback is that it requires two power supplies. Two key equations for analysis of this type of bias circuit are shown below. With these two currents known we can apply Ohm s law and Kirchhoff's law to solve for the voltages. Approximation calculation: V E = V B -V BE -1 V V E = I E R E + V EE I C I E = (-V EE - 1)/R E independent of Precise calculation: I B I E / I C I E (-V EE -V BE )/(R E + R B / DC ) 13
Emitter Bias 14
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Base Bias This type of circuit is very unstable since its changes with temperature and collector current. Base biasing circuits are rarely used in linear circuits and mainly limited to switching applications. KVL around base and collector circuits; -V CC +I B R B +V BE =0 I B = (V CC -V BE )/R B I C = DC I B I C = DC (V CC -V BE )/R B dependent of V CE = V CC -I C R C 18
Example Determine how much Q-point(I C and V CE ) for the given circuit will change a over temperature range where DC increases from 100 to 200. Assume V BE = 0.7 V. 19
Review Summary(1) The purpose of biasing is to establish a stable operating point (Qpoint). The Q-point is the best point for operation of a transistor for a given collector current. The dc load line helps to establish the Q-point for a given collector current. The linear region of a transistor is the region of operation within saturation and cutoff. 20
Review Summary (2) Voltage-divider bias is most widely used because it is stable and uses only one voltage supply. Base bias is very unstable because it is dependent. Emitter bias is stable but require two voltage supplies. Collector-feedback is relatively stable when compared to base bias, but not as stable as voltage-divider bias. 21
Summary of Bias Circuits(1) Voltage Divider 22
Summary of Bias Circuits(2) Emiiter Bias Base Bias 23
Summary of Bias Circuits(3) Collector-feedback Bias Emitter-feedback Bias 24
BJT Amplifier One of the primary uses of a transistor is to amplify ac signals. This could be an audio signal or perhaps some high frequency radio signal. It has to be able to do this without distorting the original input. For the analysis of transistor circuits from both dc and ac perspectives, the ac subscripts are lower case and italicized. Instantaneous values use both italicized lower case letters and subscripts. 25
Linear Amplifier Recall from the previous chapter that the purpose of dc biasing was to establish the Q-point for operation. The collector curves and load lines help us to relate the Q-point and its proximity to cutoff and saturation. The Q-point is best established where the signal variations do not cause the transistor to go into saturation or cutoff. What we are most interested in is the ac signal itself. Since the dc part of the overall signal is filtered out in most cases, we can view a transistor circuit in terms of just its ac component. 26
The boundary between cutoff and saturation is called the linear region. A transistor which operates in the linear region is called a linear amplifier. Note that only the ac component reaches the load because of the capacitive coupling and that the output is 180º out of phase with input. No distortion Note. Subscript Q represents Q-point 27
I c(sat) AC load line V ce(cutoff) Graphical operation of the amplifier showing the variation of the base current, collector current, and collectorto-emitter voltage about their dc Q-point values. I b and I c are on different scales. 28
Example The ac load line of a certain amplifier extends 10µA above and below Q-point base current value of 50µA as shown. Determine the resulting peak-peak value of I c and V ce.
Common-Emitter Amplifier The CE configuration has the emitter as the common terminal, or ground, to an ac signal. The common-emitter amplifier exhibits high voltage and current gain. The output signal is 180º out of phase with the input. Note. The circuit analysis can be done by dc and ac analysis 30
CE Amplifier DC Analysis The dc component of the circuit sees only the part of the circuit that is within the boundaries of C 1, C 2, and C 3 as the dc will not pass through these components. The equivalent circuit for dc analysis is shown. The methods for dc analysis are just are the same as dealing with a voltage-divider circuit. Stiff voltage divider or not? [R IN(BASE) > 10R 2 ] 31
Example Draw waveforms of V B,V E,I E,V CE 32
Darlington Amplifier The darlington pair is used to boost the input impedance to reduce loading of high output impedance circuits. The collectors are joined together and the emitter of the input transistor is connected to the base of the output transistor. The input impedance can be determined the formula below. R in = ac1 ac2 R e 33
A darlington emitter-follower used as a buffer between a common-emitter amplifier and a low-resistance load such as a speaker.
Multistage Amplifier Two or more amplifiers can be connected to increase the gain of an ac signal. The overall gain can be calculated by simply multiplying each gain together. A v = A v1 A v2 A v3 Reminder: The term GAIN is used to describe the amplification capability. 35
Decibels Amplifier voltage gain is often expressed in decibels(db) A v(db) = 20logA v Each stage s gain can now can be simply added together for the total. 36
The capacitive coupling keeps dc bias voltages separate but allows the ac to pass through to the next stage. 37
Direct coupling(no coupling or by pass capacitor) between stage improves low frequency gain. The disadvantage is that small changes in dc bias from temperature changes or supply variations becomes more pronounced. Application: Low frequency or dc(0 Hz) amplifier 38
Example 1. Whether or not this voltage-divider is stiff? 2. Determine V B V E I E I C and V C 39
Example 1. Whether or not this voltage-divider is stiff? 2. Determine V B V E I E I C V C and V CE 40
Differential Amplifier Outputs are a function of the difference between two inputs 2 operational modes: differential mode and common mode It is important as a basic of operational amplifiers.(op-amp) 41
Basic Differential Amp 42
Single-ended input operation of differential amplifier
Double-ended input operation of differential amplifier
Common-mode operation of differential amplifier
Example Draw the waveforms of Vout1 and Vout2 47
CMRR Common-mode rejection ratio(cmrr) is the measure of an amplifier s ability to reject common-mode signal. It is defined as a ratio of differential voltage gain and common-mode gain. The higher CMRR, the better. CMRR A v( d ) A cm CMRR 20log( A v( d ) A cm ) (in decibel) 48
Summary Most transistors amplifiers are designed to operate in the linear region. Transistor circuits can be view in terms of its ac equivalent for better understanding. The common-emitter amplifier has high voltage and current gain. The common-collector has a high current gain and voltage gain of 1. It has a high input impedance and low output impedance. The common-base has a high voltage gain and a current gain of 1. It has a low input impedance and high output impedance Multistage amplifiers are amplifier circuits cascaded to increased gain. We can express gain in decibels (db). 49
Supplement
Loading Effect V in R total R o + V o - Load side R load The load resistance connected to the potentiometer introduce the error to the output voltage V R R [Unloaded] Without R load, o in [Load] With R load, V o R o o total // R V load ( R R ) R // R total o o load Loading Error: V [ unloaded ] V [ load ] o o V in 51
Exercise From a given circuit, the output voltage from the potentiometer is used to drive the load with 100kΩ resistance. As a result, the loading error is applied at the output of the pot. What is the value of loading error in volts??? Explain the method to overcome this problem?? 52
Troubleshooting Shown is a typical voltage divider circuit with correct voltage readings. Knowing these voltages is a requirement before logical troubleshooting can be applied. We will discuss some of the faults and symptoms. 53
R1 Open With no bias the transistor is in cutoff. Base voltage goes down to 0 V. Collector voltage goes up to 10 V(V CC ). Emitter voltage goes down to 0 V. 54
Resistor R E Open: Transistor is in cutoff. Base reading voltage will stay approximately the same. Collector voltage goes up to 10 V(V CC ). Emitter voltage will be approximately the base voltage +.7 V. 55
Base Open Internally: Transistor is in cutoff. Base voltage stays approximately the same. Collector voltage goes up to 10 V(V CC ). Emitter voltage goes down to 0 V. 56