Upload
ngongoc
View
227
Download
6
Embed Size (px)
Citation preview
Lecture (08)BJT Amplifier 3
By:
Dr. Ahmed ElShafee
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١
The Sziklai Pair
• The Sziklai pair, is similar to the Darlington pair except that it consists of two types of transistors, an npn and a pnp.
• known as a complementary Darlington or a compound transistor
• The current gain is
• about the same as in the Darlington pair, as illustrated.
• The difference is that the Q2 base current is the Q1 collector current instead of emitter current, as in the Darlington
• arrangement.Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢
• An advantage of the Sziklai pair, compared to the Darlington, is that it takes less
• voltage to turn it on because only one barrier potential has to be overcome.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣
THE COMMON‐BASE AMPLIFIER
• The base is the common terminal and is at ac ground because of capacitor C2
• The input signal is capacitively coupled to the emitter.
• The output is capacitively coupled from the collector to a load resistor.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٤
Voltage Gain
• Vin = Ve, Vout = Vc
•
•
•
•
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٥
Input Resistance
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٦
Output Resistance
•
•
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٧
Current Gain
•
•
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٨
Power Gain
•
•
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٩
Example 07
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٠
Example 07
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١١
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٢
Multistage Amplifier
• The overall voltage gain A’v, of cascaded amplifiers,
• Amplifier voltage gain is often expressed in decibels (dB) as follows:
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٣
• This is particularly useful in multistage systems because the overall voltage gain in dB is the sum of the individual voltage gains in dB
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٤
Example 01
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٥
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٦
Capacitively‐Coupled Multistage Amplifier• we will use the two‐stage capacitively coupled amplifier in
Figure
• The output of the first stage capacitively coupled to the input of the second stage.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٧
• Capacitive coupling prevents the dc bias of one stage from affecting that of the other but allows the ac signal to pass without attenuation XC==0
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٨
Loading Effects
• the total input resistance of the second stage presents an ac load to the first stage.
• the effective ac collector resistance of Q1 is the total of all these resistances in parallel (R3, R5, R6, and Rin(base2))
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II١٩
• The ac collector resistance of the first stage is
•
• Therefore, the base‐to‐collector voltage gain of the first stage is
• Voltage Gain of the Second Stage; The second stage has no load resistor, so the ac collector resistance is R7, and the gain is
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٠
• Overall Voltage Gain
• If an input signal of 100uv , then output voltage is = (100 mV)(13,495) =1.35 V
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢١
DC Voltages in the CapacitivelyCoupled Multistage Amplifier• Since both stages are identical, the dc voltages for Q1 and Q2
are the same.
• βDCR4 >> R2, βDCR8 >> R6
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٢
Direct‐Coupled Multistage Amplifiers• no coupling or bypass capacitors in this circuit.
• The dc collector voltage of the first stage provides the base‐bias voltage for the second stage.
• Because of the direct coupling, this type of amplifier has a better low‐frequency response (Direct‐coupled amplifiers can be used to amplify low frequencies all the way down to dc (0 Hz) ) than the capacitively coupled.
• The increased reactance of capacitors at lower frequencies produces gain reduction in capacitively coupled amplifiers.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٣
differential amplifier (diff‐amp)
1. • when both inputs are grounded (0
V), the emitters are at ‐0.7V.
• It is assumed that the transistors are identically matched by careful process control during manufacturing so that their dc emitter currents are the same when there is no input signal.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٤
• Since both collector currents and both collector resistors are equal
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٥
2• Next, input 2 is left grounded, and
a positive bias voltage is applied to input 1,
• The positive voltage on the base of Q1 increases IC1 and raises the emitter voltage to
• This action reduces the forward bias (VBE) of Q2 because its base is held at 0 V (ground), IC2 decrease
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٦
• increase in IC1 causes a decrease in VC1,
• and the decrease in IC2 causes an increase in VC2,
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٧
3• Finally, input 1 is grounded and a
positive bias voltage is applied to input 2.
• Applied VB2, increases IC2, decrease in VC2, VE2 voltage is raised.
• reduces the forward bias of Q1,
• decrease in IC1 causes VC1 to increase
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٨
Modes of Signal Operation
• Single‐Ended Differential Input:
• one input is grounded and the signal voltage is applied only to the other input,
• inverted, amplified signal voltage appears at output 1 as shown
• a signal voltage appears in phase at the emitter of Q1
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٢٩
• Q1 and Q2 are common, emitter signal becomes an input to Q2, which functions as a common‐base amplifier
• at Q2 output 2; amplified noninverted signal.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٠
• In the case where the signal is applied to input 2 with input 1 grounded, an inverted, amplified signal voltage appears at output 2, noninverted, amplified signal appears at output 1.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣١
• Double‐Ended Differential Inputs:
• two opposite‐polarity (out‐of‐phase) signals are applied to the inputs
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٢
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٣
• Common‐Mode Inputs: two signal voltages of the same phase, frequency, and amplitude are applied to the two inputs
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٤
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٥
• This action is called common‐mode rejection.
• Common‐mode rejection means that this unwanted signal will not appear on the outputs and distort the desired signal.
• Common‐mode signals (noise) generally are the result of the pick‐up of radiated energy on the input lines from adjacent lines, the 50 Hz power line, or other sources.
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٦
Common‐Mode Rejection Ratio
• Desired signals appear on only one input or with opposite polarities on both input lines.
• These desired signals are amplified and appear on the outputs as previously discussed.
• Unwanted signals (noise) appearing with the same polarity on both input lines are essentially cancelled by the diff‐amp and do not appear on the outputs. The measure of an amplifier’s
• ability to reject common‐mode signals is a parameter called the CMRR (commonmode rejection ratio).
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٧
• Ideally, a diff‐amp provides a very high gain for desired signals (single‐ended or differential) and zero gain for common‐mode signals, practically is a very small value
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٨
Example 02
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٣٩
•
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٤٠
Thanks,..
See you next week (ISA),…
Dr. Ahmed ElShafee, ACU : Fall 2016, Electronic Circuits II٤١