Lab Experiment: Differential pair amplifier
Contents
Aim of the lab exercise
The described lab exercise takes place in the curriculum of
electrical engineers. The topic is analogue electronics. The
objective of the lab is to familiarise the students with the
caracteristics of different architectures for the differential
paire.
Emitter and source coupled pairs are certainly the most
widely used two-transistor subcircuits in analogue circuit
design. It is of particular interest to investigate their
characteristics in lab exercises.
Keywords
emitter coupled differential pair
active and passive load
active current source
differential mode
common mode
common mode rejection ratio
output impedance
1 Prelab
We want to investigate an emitter coupled differential pair
amplifier. The transistors under test have the following figures of
merit:
| |
npn
| pnp
|
| Early Voltage (V)
| 70 |
60 |
| Current gain
| 100 |
50 |
The small signal equivalent circuit is represented in figure 1.
Figure 1: Small signal equivalent circuit for the bipolar transistor
The power supply VCC is set to 10V.
| R1 = R2 = | 6.8 kΩ
|
| RS = | 7.1kΩ
|
| VBE = | 0.7 V
|
1.1 Basic differential pair
We are starting with the circuit from figure 2:
Figure 2: emitter coupled pair with long tail current source and passive load
- Identify the different blocs from this circuit.
- DC operating point:
- Calculate the quiescent current IRS. The DC part of V1 and V2 is 0.
- Annotate all DC voltages at all nodes and the branch currents within this circuit.
The transistors Q1 and Q2 are supposed to be identical.
-
Differential mode: V1 = -V2
- Plot the small signal equivalent circuit. Remark:
You will find a virtual ground in the differential mode.
- Give the expression for and calculate the voltage
gain and the output impedance.
- Common mode: V1 = V2
- Plot the small signal equivalent circuit. (neglect rce1 and rce2)
- Give the expression for and calculate the common mode voltage gain.
- Calculate the common mode rejection ratio.
1.2 Basic differential pair, active current source
The common mode rejection ratio can be improved when replacing
the long tail resistance RS by an active current source (figure 3).
Figure 3: emitter coupled pair with active current source and passive load
We want to use for the differential pair the same current as
before.
- Calculate the corresponding value of Rpol ?
- Annotate all
DC voltages at all nodes and the branch currents.
- Is the
differential mode behaviour of the amplifier with active current
source different of an amplifier with long tail resistance as
current source?
- Plot the small signal equivalent circuit to
calculate the output impedance ZS of the active current source.
-
Calculate ZS.
- Calculate the common mode voltage gain. Simplify
the expression.
- Calculate the common mode rejection
ratio. Compare with the previous result.
1.3 Differential pair, active load and active current source
Improvement of the differential mode voltage gain:
The resistors R1 and R2 are replaced by an active load Q5 and
Q6 (figure 4).
Figure 4: emitter coupled pair with active current source and active load
- Plot the small signal equivalent circuit for the
differential mode.
- Calculate the parameter values of rbe5, rbe6,
rce5, rce6, gm5, gm6.
- Give the expression for and calculate the
voltage gain and the output impedance.
2 Simulation
(not yet available)
3 Experiments
The subject of this lab exercise consists in investigating different
architectures of emitter coupled pairs. For these architecture the
common mode caracteristics as well as the differential mode
caracteristics are analysed. The output impedance is also
determined for all cases.
3.1 Emitter coupled pair with long tail current source and passive load
3.1.1 Differential mode
The hardware implementation is shown on figure 5 for the emitter
coupled pair with long tail current source and passive load. The
inverter with the variable resistor assures that V1 = -V2.
Figure 5: Hardware implementation of the emitter coupled pair with long tail
current source and passive load (differential mode)
The value of the components are:
| RC = | 4,7 kΩ | | C = | 1 µF
|
| RE = | 4,7 kΩ | V+ = | 10V
|
| R' = | 1kΩ | V- = | -10V
|
| P = | 1kΩ
|
Analysis in the frequency domain
(
info),
Perform a measurement.
Attention: keep VOSC under 8mV and the maximum frequency
below 100kHz !!!
- Measure the low frequency differentiel voltage gain (no load at the output).
3.1.2 Common mode
Figure 6: Hardware implementation of the emitter coupled pair with long tail
current source and passive load (common mode)
| RC = | 4,7 kΩ | | V+ = | 10V
|
| RE = | 4,7 kΩ | V- = | -10V
|
| C = | 1 µF
|
Analysis in the time domain (info):
Perform a measurement.
-
Verify the previous result using the measure in the time
domain.
-
Measure the input common mode dynamic range.
3.2 Emitter coupled pair with active current source and passive load
3.2.1 Differential mode
Figure 7: Hardware implementation of the emitter coupled pair with active current
source and passive load (differential mode)
RC = 4,7 kΩ
R = 10kΩ
R' = 1kΩ
P = 2,2kΩ
| C = 1 µF
V+ = 10V
V- = -10V
|
Analysis in the frequency domain (
info):
Perform a measurement.
Attention: keep VOSC under 8mV and the maximum frequency
below 100kHz !!!
- Measure the low frequency differentiel voltage gain (no load at the output).
3.2.2 Output impedance
Figure 8: Hardware implementation of the emitter coupled pair with active current
source and passive load (output impedance determination)
RC = 4,7 kΩ
R = 10kΩ
|
RL =4,7 kΩ
R' = 1kΩ
| P = 1kΩ
C = 1 µF
| V+ = 10V
V- = -10V
|
Analysis in the frequency domain (info):
Perform
a measurement.
Attention: keep VOSC under 30mV and the maximum frequency
below 100kHz !!!
- Measure the low frequency differentiel voltage gain (with load RL at
the output).
- From the unloaded voltage gain, determine output resistance.
3.2.3 Common mode
Figure 9: Hardware implementation of the emitter coupled pair with active current
source and passive load (common mode)
RC = 4,7 kΩ
R = 10kΩ
C = 1 µF | V+ = 10V
V- = -10V
|
Analysis in the time domain (info):
Perform a
measurement.
- Verify the previous result using the measure in the time domain.
- Measure the input common mode dynamic range.
3.3 Emitter coupled pair with active current source and active load
3.3.1 Differential mode
Figure 10: Hardware implementation of the emitter coupled pair with active current
source and active load (differential mode)
R = 10kΩ
Ra= 1kΩ
| Rb = 1kΩ
C = 1 µF | V+ = 10V
V- = -10V
|
Analysis in the frequency domain (info):
Perform
a measurement.
Attention: keep VOSC under 0.35mV and the maximum
frequency below 100kHz !!!
- Measure the low frequency differentiel voltage gain (no load at the output).
3.3.2 Output impedance
Figure 11: Hardware implementation of the emitter coupled pair with active current
source and active load (output impedance determination)
R = 10kΩ
RL = 10 kΩ
Ra =
1kΩ
Rb = 2,2kΩ | C = 1 µF
V+ = 10V
V- = -10V |
Analysis in the frequency domain (info):
Perform
a measurement.
Attention: keep VOSC under 1.6mV and the maximum
frequency below 100kHz !!!
- Measure the low frequency differentiel voltage gain (with load RL at
the output).
- From the unloaded voltage gain, determine output resistance.
3.3.3 Common mode
Figure 12: Hardware implementation of the emitter
coupled pair with active current source and active load (common
mode)
R = 10kΩ
P1 = 1kΩ
C = 1 µF | V+ = 10V
V- = -10V
|
Analysis in the time domain (info):
Perform a
measurement.
- Verify the previous result using the measure in the time domain.
- Measure the input common mode dynamic range.
4 Discussion
- Compare theory and experiment.
- Propose improvements for the current source.
- What is the origin of the common mode voltage gain ?
- Does the common mode voltage gain still exist when the current source is ideal
?
- Why do we want to cancel the common mode voltage gain?
- How can the linear range of the emitter coupled pair be increased?
- How can the input impedance of the emitter coupled pair be increased?