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By:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١
Lecture (04)Transistor Bias Circuit ‐3
BJT Amplifiers 1
Emitter‐Feedback Bias
• If an emitter resistor is added to the base‐bias circuit in Figure, the result is emitter feedback bias.
• If the collector current tries to increase, the emitter voltage increases, causing an increase in base voltage because VB =VE + VBE.
• This increase in base voltage reduces the voltage across RB, thus reducing the base current and keeping the collector current from increasing.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢
• A similar action occurs if the collector current tries to decrease.
• While this is better for linear circuits than base bias, it is still dependent on βDC and is not as predictable as voltage‐divider bias.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣
• Kirchhoff’s voltage law
•
IE= Ic =
KVL
VCC ‐ IC RC – VCE ‐ IE RE = 0
VCE=VCC – IC RC – RE IC
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤
Example 04
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٦
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٧
Collector‐Feedback Bias
• The collector voltage provides the bias for the base‐emitter junction.
• The negative feedback creates an “offsetting” effect that tends to keep the Q‐point stable.
• When VC decreases, there is a decrease in voltage across RB, which decreases IB.
• The decrease in IB produces less IC which, in turn, drops less voltage across RC and thus offsets the increase in VC.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٨
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٩
• Ohm’s law
• Let’s assume that IC>>IBThe collector voltage is
• Substituting
• The terms can be arranged
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٠
• solve for IC
• Since the emitter is ground, VCE = VC
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١١
• Q‐Point Stability Over Temperature
• Equation shows that the
collector current is dependent to some extent on β, VBE
• This dependency, increase if RC>>RB/β, VCC>>VBE
• β varies directly with temperature, VBE varies inversely with temperature.
• As the temperature goes up in a collector‐feedback circuit, β goes up and VBE goes down
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٢
• The increase in β acts to increase IC.
• The decrease in VBE acts to increase IB which, in turn also acts to increase IC.
• As IC tries to increase, the voltage drop across RC also tries to increase.
• This tends to reduce the collector voltage and therefore the voltage across RB, reducing IB and offsetting the attempted increase in IC and the attempted decrease in VC.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٣
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٤
Example 05
Calculate the Q‐point values (IC and VCE) for the circuit in Figure
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٥
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II١٦
summery
• Emitter Bias
• Emitter Bias
• Base Bias
• Emitter‐Feedback Bias
• Collector‐Feedback Bias
The Linear Amplifier
• A linear amplifier provides amplification of a signal without any distortion so that the output signal
• A voltage‐divider biased transistor with a sinusoidal ac source capacitively coupled to the base through C1 and a load capacitively coupled to the collector through C2 is shown in Figure
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٠
• The collector current varies above and below its Q‐point value, ICQ, in phase with the base current.
• The sinusoidal collector‐to‐emitter voltage varies above and below its Q‐point value, VCEQ, 180 out of phase with the base voltage
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢١
• The operation just described can be illustrated graphically on the ac load line, as shown
• sinusoidal voltage at the base produces a base current that varies above and below the Q‐point on the ac load line
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٢
• The ac load line differs from the dc load line because the effective
• ac collector resistance is RL in parallel with RC and is less than the dc collector resistance RC alone.
Example 01
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٣
• collector current varying from 6 mA to 4 mA for a peak‐to‐peak value of 2 mA and the collector‐to‐emitter voltage varying from 1 V to 2 V for a peak‐to‐peak value of 1 V.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٤
TRANSISTOR AC MODELS
• The five r parameters commonly used for BJTs are
• (r ‘b); small enough to neglect
• (r’c); several hundred kilohms and can be replaced by an open.
• The collector effectively acts as a dependent current source of
• or, equivalently ,Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٥
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٦
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٧
• assuming an abrupt junction between the n and p regions. It is also temperature dependent and is based on an ambient temperature of 20°C.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٢٨
Example 02
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٠
Comparison of the AC Beta ( βac) to the DC Beta (βDC)• For a typical transistor, a graph of IC versus IB is nonlinear, as
shown in Figure.
• If you pick a Q‐point on the curve and cause the base current to vary an amount Δ IB then the collector current will vary an Δ IC amount as shown in part (b).
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣١
• At different points on the nonlinear curve, the ratio Δ IC / ΔIBwill be different, and it may also differ from the IC /IB ratio at the Q‐point.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٢
h Parameters
• A manufacturer’s datasheet typically specifies h (hybrid) parameters (hi, hr, hf, and ho)
• Basic ac h parameters.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٣
• Subscripts of h parameters for each of the three amplifier configurations
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٤
Relationships of h Parameters and r Parameters
• datasheets often provide only common‐emitter h parameters, the following formulas show how to convert them to r parameters.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٥
THE COMMON‐EMITTER AMPLIFIER• common‐emitter amplifier with voltage‐divider bias and
coupling capacitors C1 and C3 on the input and output and a bypass capacitor, C2, from emitter to ground.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٦
• There is no signal at the emitter because the bypass capacitor effectively shorts the emitter to ground at the signal frequency.
• All amplifiers have a combination of both ac and dc operation, which must be considered, but keep in mind that the common‐emitter designation refers to the ac operation.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٧
• Phase Inversion The output signal is 180° out of phase with the input signal.
• As the input signal voltage changes, it causes the ac base current to change, resulting in a change in the collector current from its Q‐point value.
• If the base current increases, the collector current increases above its Q‐point value, causing an increase in the voltage drop across RC.
• This increase in the voltage across RC means that the voltage at the collector decreases from its Q‐point.
• So, any change in input signal voltage results in an opposite change in collector signal voltage, which is a phase inversion.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٨
DC Analysis
• a dc equivalent circuit is developed by removing the coupling and bypass capacitors
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٣٩
• circuit can be redrawn
• Calculate VTH, RTH
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٠
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤١
AC Analysis
• The capacitors C1, C2, and C3 are replaced by effective shorts because their values are selected so that XC is negligible at the signal frequency and can be considered to be 0 ohm
• The dc source is replaced by ground
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٢
• is called a common‐emitter amplifier because the bypass capacitor C2 keeps the emitter at ac ground
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٣
• If the internal resistance of the ac source is then all of the source voltage appears at the base terminal.
• If, however, the ac source has a nonzero internal resistance, then three factors must be taken into account in determining the actual signal voltage at the base.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٤
• R1, R2, and Rin(base) in parallel to get the total input resistance, Rin(tot),
• A high value of input resistance is desirable so that the amplifier will not excessively load the signal source.
• This is opposite to the requirement for a stable Q‐point, which requires smaller resistors.
• many trade‐offs that must be considered when choosing components for a circuit.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٥
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٦
• Vs, is divided down by Rs (source resistance) and Rin(tot) so
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٧
• use the simplified r‐parameter model of the transistor.
• Figure shows the transistor model connected to the external collector resistor, RC
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٨
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٤٩
• The output resistance of the common‐emitter amplifier is the resistance looking in at the collector
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• since the internal ac collector resistance of the transistor, is typically much larger than RC, the approximation is usually valid
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٠
Example 03
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥١
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Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٢
Voltage Gain
• The gain is the ratio of ac output voltage at the collector (Vc) to ac input voltage at the base (Vb).
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٣
Attenuation
• is the reduction in signal voltage as it passes through a circuit and corresponds to a gain of less than 1.
• if the signal amplitude is reduced by half, the attenuation is 2, which can be expressed as a gain of 0.5.
• Suppose a source produces a 10 mV input signal and the source resistance combined with the load resistance results in a 2 mV output signal. In this case, the attenuation is 10 mV/2 mV = 5
• gain as 1 /5 =0.2.
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٤
• The overall voltage gain of the amplifier
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٥
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Fall 2018, Electronic Circuits II٥٦