Lecture (04) Transistor Bias Circuit 3 BJT Amplifiers 1 By: Dr. Ahmed ElShafee ١ 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. ٢
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. ٣ Kirchhoff s voltage law KVL I E = I c = V CC I C R C V CE I E R E = 0 V CE =V CC I C R C R E I C ٤
Example 04 ٥ ٦
٧ 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. ٨
٩ Ohm s law Let s assume that I C >>I B The collector voltage is Substituting The terms can be arranged ١٠
solve for IC Since the emitter is ground, VCE = VC ١١ Q Point Stability Over Temperature Equation shows that the collector current is dependent to some extent on β, V BE This dependency, increase if R C >>R B /β, V CC >>V BE β varies directly with temperature, V BE varies inversely with temperature. As the temperature goes up in a collectorfeedback circuit, β goes up and VBE goes down ١٢
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. ١٣ ١٤
Example 05 Calculate the Q point values (IC and VCE) for the circuit in Figure ١٥ ١٦
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 ٢٠
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 ٢١ 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 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. ٢٢
Example 01 ٢٣ collector current varying from 6 ma to 4 ma for a peak topeak 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. ٢٤
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, ٢٥ ٢٦
٢٧ 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. ٢٨
Example 02 ٢٩ ٣٠
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). ٣١ At different points on the nonlinear curve, the ratio Δ IC / ΔIB will be different, and it may also differ from the IC /IBratio at the Q point. ٣٢
h Parameters A manufacturer s datasheet typically specifies h (hybrid) parameters (hi, hr, hf, and ho) Basic ac h parameters. ٣٣ Subscripts of h parameters for each of the three amplifier configurations ٣٤
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. ٣٥ 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. ٣٦
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. ٣٧ 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. ٣٨
DC Analysis a dc equivalent circuit is developed by removing the coupling and bypass capacitors ٣٩ circuit can be redrawn Calculate V TH, R TH ٤٠
٤١ 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 ٤٢
is called a common emitter amplifier because the bypass capacitor C2 keeps the emitter at ac ground ٤٣ 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. ٤٤
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. ٤٥ ٤٦
Vs, is divided down by Rs (source resistance) and Rin(tot) so ٤٧ use the simplified r parameter model of the transistor. Figure shows the transistor model connected to the external collector resistor, RC ٤٨
٤٩ The output resistance of the commonemitter amplifier is the resistance looking in at the collector since the internal ac collector resistance of the transistor, is typically much larger than RC, the approximation is usually valid ٥٠
Example 03 ٥١ ٥٢
Voltage Gain The gain is the ratio of ac output voltage at the collector (Vc) to ac input voltage at the base (Vb). ٥٣ 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. ٥٤
The overall voltage gain of the amplifier ٥٥ Thanks,.. See you next week (ISA), ٥٦