And etc
Learning Objectives
- To understand how to use Multisim.
- This experiment will help me to understand the output characteristics of a bipolar junction transistor.
- To understand the input characteristics.
- Will help to understand transistor biasing.
- To check bias condition.
- How to set up instruments and measuring the gain.
- The effect of load variation.
- The frequency response.
- The effect of Ce.
- To understand the functions of different types of amplifiers.
Background theory
Bipolar junction transistor can be used in a common emitter amplifier as shown below:
The diagram above is the variation of the collector current with collector-emitter voltage for different values of base current. To achieve a linear amplification of signals using a d.c current level is set through the base (Ibq) and the a.c signal is added to this so that the total base current goes between values Ib1 and Ib2. Therefore the d.c current through the base is known as a bias current.
For a bias circuit
Connecting the bias resistor to the collector
The transistor is a non-linear circuit element, Ic is not directly proportional to Vce and ohm’s law cannot be applied to only the transistor.
For the common- emitter configuration the output characteristics are a plot of the output current (Ic) versus the output voltage (Vce) for a range of values of input current (Ib) as shown below:
The input characteristics are a plot of input current (Ib) versus the input voltage (Vbe) for a range of output voltages (Vce)
The active region for the common-emitter configuration is that portion of the upper-right quadrant that has the greatest linearity which is the region in which the curves of Ib are nearly equally spaced and straight.
The active region of the common emitter configuration can be employed for voltage, power amplification and current.
The cut of region for the common emitter configuration is not well defined compared to the common base configuration. The collector characteristics of Ic is not equal to zero when Ib is zero.
The operating mode of a bipolar junction transistor depends on its junction’s active mode in applications where it is used as an amplifier. When it enters cut off or saturation modes, it behaves like an open or close switch respectively.
When choosing an operating point, temperature variations have to be considered.
Forward-active or active is when the emitter-base junction is forward biased and the base-collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, βf in forward-active mode. If this is the case, the collector-emitter current is approximately to the base current, but a lot larger, for small base current variations.
Reverse-active or inverse-active or inverted can be done by reversing the biasing conditions of the forward-active region theb the bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Since most BJTs are designed to maximise current gain in forward-active mode, the βf in inverted mode is several (2 - 3 for the ordinary germanium transistor) times smaller.
Saturation occurs when both junctions are forward-biased which makes the BJT in saturation mode. This will make it facilitate high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch.
Cutoff is a biasing condition opposite to that of saturation (with both junctions reverse biased) are present. There will be a small current flow which will correspond to a logical “off”, or an open switch
A bipolar junction transistor can be connected to form a single stage amplifier in the common emitter configuration shown below using a NPN transistor
When the circuit above is used as an amplifier, there will be a shortage in gain at low frequecies because of the input capacitor and the capacitor Ce. There is also a shortage in gain at high frequecies because of the characteristics of the transistor. The variation of gain with frequenct will have characteristics as shown below
The frequencies f1 and f2 are known as the upper and lower frequency cut-off frequencies for the amplifier.
The bandwidth of the amplifier = f2 - f1 Hz.
An inverting amplifier: The gain of the op-amp is infinite if the output voltage is finite so therefore the input voltage to the op amp (V+-V_) must be zero. Therefore
V_=V+=0
In this circuit, the negative feedback maintains the voltage on the inverting input (V_) at zero volts. This may be understood by noting that V_ becomes more positive than the voltage on the non-inverting input( in this case zero volts), this will cause the output of the op-amp to become negative, which will drive V_ more positive. Therefore the circuit will act to keep V_ at zero, even though this terminal is not physically connected to earth. Such a point in a circuit is referred to as a virtual earth, and this kind of amplifier is a called a virtual earth amplifier.
A non inverting amplifier
The gain of the op-amp (V+ - V_) must be zero. So therefore:
V_= V+ =Vi
Since the op-amp has an infinite input resistance,it input current must be zero. Therefore, V_ is determined simply by the output voltage and the potential divider formed by R1 and R2 therefore:
The summer-inverter
The circuit is smiliar in form to the inverting amplifier with the addition of an extra input resistor. As for the earlier circuit, the inverting input to the op-amp forms a virtual earth and therefore V_ is zero. This makes the various currents in the circuit easy to calculate.
the output consists of the inverted sum of these weighted inputs and can be calculated using the following formula
The unity gain buffer
This is a special case of the non inverting amplifier with R1 equal to zero and R2 equal to infinity.
the importance of this circuit is that it has a very high input resistance and a very low output resistance, making it very useful as a buffer.
Integrator
It integrates the inverted signal over time.
An ideal integrator doesn’t require resistor R1 but for a non but for a non ideal case a resistor is added in parallel to the capacitor to prevent the integrator from becoming saturated.
The differentiator circuit
Since no current flows into the inputs of the op-amp, the voltages on the two inputs are determined simply by the potential divider formed by external resistors.
It produces an output voltage proportional to the differential of its input voltage. Connecting the circuit using a capacitor and resistors as shown below:
The circuit above is not an ideal differentiator because an ideal circuit would not have R2 but it’s impractical. The differentiator circuit acts as the inverting amplifier after a frequency (f1) is specified by the component C1 and R2 as shown in the graph above.
Method
Apparatus: Electronics Workbench (Multisim) program running on an MSWindows PC.
Experiment 1
Output Characteristics
- Input the circuit schematic shown below to Multisim and save the file
- plot each output characteristic by setting a voltage on Vb and then plotting the collector current (V3) for various values of Vc.
- From the stimulate menu select Analyses and then DC Sweep for parameters shown below
- Click the output tab in the circuit box. Select the output that corresponds to the circuit designed
- Click stimulate
Input characteristics
- Draw the circuit shown below
- set the DC sweep analysis to the values shown below
- Select the output that corresponds to the node number for the base of the transistor
- Click stimulate
Transistor Biasing
- Design a bias circuit using the following requirements
I. VCC is 15V.
II. IC must be 100mA.
III. RC is 50Ω.
(2) Check bias point then select Simulate > Analyses > DC Operating Point. Select the voltages applicable to
Vcc, Vbe and Vce and then click on Simulate.
(3) Measure the thermal effects by select Simulate > Analyses > Temperature Sweep. Set up the Analysis
parameters as in fig. 11. Then:
• Click on the ‘More>>’ button. In the ‘Analysis to Sweep’ box, select ‘DC Operating Point’.
• Make sure that ‘Group all traces in one plot’ remains selected.
• On the Output tab, select the voltage that corresponds to Vce
(4)Edit the values as shown below
Experiment 2
- connect the ELE303 Lab2 board as shown below
(2)Bias condition check: Before applying an input signal check the bias circuit for the transistor by measuring and noting the d.c value of Vce. Use the DMM and touch the test leads against the two 470 ohm resistors.
(3) Setting up the instruments and measuring the gain: connect the signal generator between the input and ground. Set the output of the signal generator at 4Khz and around 100mV peak to peak. Connect channel A of the oscilloscope to the input signal and channel B across the load resistor.
(4) Effect of load variation: vary the value of Rl between 500Ω and 2KΩ and plot the voltage gain at 4Khz against the total load resistance.
(5) Frequency response: Measure the input and output voltages for frequencies from 10Hz to 1MHz with Rl set st 1kΩ and plot the voltage gain against the frequency
(6) Effect of Ce: remove the emitter from the circuit and repeat the frequency response plot
Discussion (Results)
Output Characteristics
Using the equation Ib=(Vb-0.7)/R1 for values of Vb(1,2,3,……volts)
The output characteristics
Input Characteristics
Transistor Biasing
The designed bias circuit is shown below:
The values gotten from the stimulation is not the same as the values calculated because in the calculation we used 0.7 and in stimulation we used 0.77305272
The calculation
Ic=Hfe Ib
100 x 10^-3=156Ib
Ib=0.64mA
Ib=Vcc-0.7/Rb
0.64mARb=14.3
Rb=22.34KΏ
From the circuit
The voltage through Rc
Ic+Ib=100+0.64=100.64mA
The voltage through Rc
V=(Ic+Ib)Rc
V=100.64E-3 x 50=5.032V
Vce=Vcc-Vc
Vce=15-5.032=9.968V
The stimulation Values
Error Analysis
Percentage error(%)= error/original value x 100
% Error in Vce
9.968-9.80292/ 9.80292= 0.0168%
% Error in Vbe
0.77305272-0.7/0.77305272 x 100 = 9.449%
% Error in the Vcc
15-15/15 = 0%
The Thermal effects
Temperature Sweep
The bias condition is maintained very well regardless of the change in temperature because there is a very small change or decrease in the values. I suggest this will maintain the bias current better because an increase in temperature does not take a drastic effect on the values. An increase in current will generate more heat which will in turn increase the temperature therefore the bias circuit will be able to handle that effect.
Ic is directly proportional to Ib, so therefore an increase in the gain by 50% will cause an increase in Ic because Ic=Hfe Ib.
Stimulated results for common emitter amplifier circuit
The circuit with the common emitter
The Ac analysis with c3
Transient analysis with c3
The circuit WITHOUT C3
The Oscilloscope reading without C3
Ac Analysis without the C3 component
Transient analysis
Bias condition check
Using the DMM and touch test leads against the two 470 ohmn resistors i.e the bottom of the collector resistor and the top of the emitter resistor gave a value Vbe=0.65V and Vce=7.01V
Setting up the instruments and measuring gain
The output voltage=2.55V
Gain=Vo/Vin
Gain=2.55/100x10^-3=25.5
Effect of load variation
Frequency Response
Effect of Ce
Mid-frequency voltage gain is the maximum gain of an amplifier. This gain depends on the frequency, for an RC coupled voltage amplifier the gain is essentially equal to this value over a large range of frequencies.
The load resistance affects gain because the output voltage is affected by the load resistance as well as the characteristics of the amplifier. The lower the resistance that is applied across the output of a circuit the more heavily it is loaded since more current is drawn from it.
Inverting amplifier
The Summer-Inverter
The non inverting amplifier
Ac analysis
The unity buffer
Ac analysis
Vout=1 x0.1=0.1V
Vin-1 c 0.1=0.1V
Gain=0.1/0.1=1
The integrator
Conclusion
The experiment carried out above has helped me to achieve the learning objectives listed above.
References:
- Neil Storey (2004) ,Electrical and Electronic Systems, Prentice Hall
- Robert L Boylestad Louis Nashelsky (2006), Electrical Devices and Circuit Theory,Prentice Hall
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Mid gain frequency
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Transistors and Operational Amplifiers
- Lecture notes
- Labsheet
