EXP7: AMPLIFIERS I.
Objectives.
The Objectives of this lab are:
• To know the physical meaning of the transistors parameters as well as how to measure them.
• To understand the meaning of biasing for transistor circuits.
• To review some of the configurations to bias the transistor.
• To obtain and understand the transfer characteristic for the different configurations.
• To obtain, and understand the use of, the load line.
Introduction and Test Circuits.
This lab will give you the knowledge and understanding so you will be able to explain the behavior of the
transistor in the different configurations.
The configurations we are going to be using are:
• The inverter.
• The inverter with emitter resistor.
• The voltage follower.
• The current follower.
For the theory about the different configurations please refer to chapter 6 from Horenstein and/or chapters
4 and 5 from Jaeger.
Preparation.
Read chapter 6 from Horenstein and/or chapters 4 and 5 from Jaeger.
Procedure.
We will start with the BJT and then proceed to the mosfet.
37
BJT.
1. Parameters: The first thing we are to do is measure the important parameters for the BJT. Before
doing something get the VI characteristic of your transistor using BJT IVcurve.vi. Since some of them
require the operating point to be set up first, we will use 1kΩ resistor as the collector resistor and
the base current needed to place the operating point in the middle of the constant current region. To
measure this parameters we will use the inverter configuration, the circuit to implement is shown in
figure 7-1.
Figure 7-1: Circuit to be used to measure some important BJT parameters.
• βf : DC current gain. This gain is defined as follows,
βf =
IC
IB
(7-1)
This parameter is found by measuring the base and collector currents value on the operating point
and using them in the above equation.
• gm : Transconductance gain. This gain is defined as follows,
gm =
4ic
∂ic
|
|
≈
∂vbe IC ,VCE
4vbe IC ,VCE
(7-2)
This parameter is found by sligthly changing the base current up and down from the operating
point while measuring the base emitter voltage and the collector current so you will have the
increment for the current and the voltage.
• βo : AC current gain. This parameter is defined as follows,
βo =
4ic
∂ic
|IC ,VCE ≈
|
∂ib
4ib IC ,VCE
(7-3)
This parameter is found by sligthly changing the base current up and down from the operating
point while measuring the collector current so you will have the increment for both currents. For
this measurement you need to keep the collector emitter voltage constant so you will have to
change the VCC value to do it.
• rbe : Input resistance, or base emitter resistance. This parameter is defined as follows,
rbe =
4vbe
∂vbe
|
≈
|
∂ib VB ,IB
4ib VB ,IB
38
(7-4)
This parameter is found by sligthly changing the base current up and down from the operating
point while measuring the base emitter voltage so you will have the increment for the current and
the voltage.
• rce : Output resistance, or collector emitter resistance. This parameter is defined as,
rce =
∂vce
4vce
|IC ,VCE ≈
|
∂ic
4ic IC ,VCE
(7-5)
This parameter is found by sligthly changing the VCC value while measuring the collector emitter
voltage and the collector current so you will have the increment for the current and the voltage.
• Vsat : Saturation voltage. The saturation voltage is the minimum voltage you will be able to see
on the collector emitter junction. To measure this value you can trace the load line on the VI
curves of your transistor or by increasing the base current untill you see no significant decrement
in the collector emitter voltage.
2. Inverter: Build the circuit shown in figure 7-2 and do the following.
Figure 7-2: BJT inverter configuration.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage needed
to place the operational point in the middle of the linear region of the transfer characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the collector emitter voltage and the collector current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the linear region.
39
Figure 7-3: BJT inverter configuration using emitter resistor.
3. Inverter with emitter resistor: Build the circuit shown in figure 7-3 and do the following.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage needed
to place the operational point in the middle of the linear region of the transfer characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the collector emitter voltage and the collector current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the linear region.
4. Voltage Follower: Build the circuit shown in figure 7-4 and do the following.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage needed
to place the operational point in the middle of the linear region of the transfer characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the collector emitter voltage and the collector current.
40
Figure 7-4: BJT voltage follower configuration.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the linear region.
5. Current follower: Build the circuit shown in figure 7-5 and do the following.
Figure 7-5: BJT current follower configuration.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
41
• Use the information you need from all of the above you have found to find the DC voltage needed
to place the operational point in the middle of the linear region of the transfer characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the collector emitter voltage and the collector current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the linear region.
JFET.
1. Parameters: The first thing we are to do is measure the important parameters for the JFET.Before
anything, get the VI characteristic of your transistor using the program FET IVcurve.vi. Since some
of the parameters require the operating point to be set up first, we will use a 10kΩ resistor as the drain
resistor and the gate source voltage needed to place the transistor in the middle of the constan current
region, the circuit to be used is shown in figure 7-6.
Figure 7-6: Circuit to be used to find some important JFET parameters.
• VT R : Threshold voltage. Minimum voltage needed in order to start seeing significant current
flowing through the drain source junction. Remember the range for a JFET is between VT R and
0.
This parameter is found using the circuit in figure 7-7 and the program tranchar.vi. The VT R
voltage can be read easily from the curve, it is the maximum voltage reached.
• IDSS : Drain current for a zero gate-source voltage.
This parameter is found by setting VGS = 0 and VDS = −VT R and measuring ID .
• K: Conductance parameter. This parameter is a function of the gate dimensions. The following
equation can be used to find the value of K.
K=
42
IDSS
VT2R
(7-6)
Figure 7-7: Circuit to find the VT R parameter.
• gm : Transconductance gain. This gain is defined as follows,
gm =
∂iD
4iD
|ID ,VGS ≈
|
∂vGS
4vGS ID ,VGS
(7-7)
This parameter is found by sligthly changing the gate-source voltage up and down from the
operating point while measuring the drain current so you will have the increment for the current
and the voltage.
• rds : Output resistance, or drain source resistance. This parameter is defined as,
rds =
∂vds
4vds
|
|
≈
∂id ID ,VDS
4id ID ,VDS
(7-8)
This parameter is found by sligthly changing the VDD value while measuring the collector emitter
voltage and the collector current so you will have the increment for the current and the voltage.
2. Inverter: Build the circuit shown in figure 7-8 and do the following.
Figure 7-8: JFET inverter configuration.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage
needed to place the operational point in the middle of the constant current region of the transfer
characteristic.
43
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the drain source voltage and the drain current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the constant current region region.
3. Inverter with emitter resistor: Build the circuit shown in figure 7-9 and do the following.
Figure 7-9: JFET inverter configuration using a source resistor.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage
needed to place the operational point in the middle of the constant current region of the transfer
characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the drain source voltage and the drain current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the constant current region.
44
Figure 7-10: JFET voltage follower configuration.
4. Voltage Follower: Build the circuit shown in figure 7-10 and do the following.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage
needed to place the operational point in the middle of the constant current region of the transfer
characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the drain source voltage and the drain current.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the constant current region.
5. Current follower: Build the circuit shown in figure 7-11 and do the following.
• Draw the load line on top of the VI characteristic.
• Use the program tranchar.vi to get the transfer characteristic of the circuit.
• Use the information you need from all of the above you have found to find the DC voltage
needed to place the operational point in the middle of the constant current region of the transfer
characteristic.
• Use the function generator as the input source for the circuit. Use the following set up.
– Offset: The DC voltage previously found.
– Amplitude: 0.1V , or the minimum required to have the offset needed.
– Frequency: 1Khz.
• Use the oscilloscope to see the input and output signals. Record the AC voltage gain and the DC
values for the drain source voltage and the drain current.
45
Figure 7-11: JFET current follower configuration.
• Increase the amplitude of the function generator until you see the output clipped at both peaks.
Then use the XY display mode on the oscilloscope, the input signal has to be in channel 1 and
the output signal in channel 2. What you expect to see here is the transfer characteristic. Use the
cursors to measure the maximum output voltage as well as the minimum output voltage. Record
the range for the input voltage where the transistor is on the constant current region region.
Analysis.
Give a mathematical description for the transfer characteristic in the constant current region for all of the
configurations you study in this experiment. Compare your results with the curves found in lab.
46