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Table of Contents
Activity 24. BJT Multivibrators
Notes:
As in all the ALM labs we use the following terminology when referring to the connections to the M1000 connector and configuring the hardware. The green shaded rectangles indicate connections to the M1000 analog I/O connector. The analog I/O channel pins are referred to as CA and CB. When configured to force voltage / measure current -V is added as in CA-V or when configured to force current / measure voltage -I is added as in CA-I. When a channel is configured in the high impedance mode to only measure voltage -H is added as CA-H.
Scope traces are similarly referred to by channel and voltage / current. Such as CA-V , CB-V for the voltage waveforms and CA-I , CB-I for the current waveforms.
In this Lab you will be using one or both of the channels of the ALM1000 in the split Input / Output mode. CB-OUT is used to denote the connection to the Output pin and CB-IN is used to denote the Input pin on the (expanded) 8 pin connector.
Background:
A multivibrator circuit consists generally of two inverting amplifier stages. The two amplifiers are connected in series or cascade, and a feedback path connects from the output of the second amplifier back to the input of the first. Because each stages inverts the signal, the overall feedback around the loop is positive. There are three main types of multivibrators. In the astable multivibrator capacitors are used to couple the two amplifier stages and provide the feedback path. Since the capacitors block any DC signals (sometimes referred to as state) from passing from one stage to the next the astable multivibrator has no stable DC operating point and is thus a free-running oscillator. In the monostable multivibrator the coupling from one of the stages to the other uses one capacitor while the second connection is through a DC path. Thus the monostable multivibrator has one stable DC stage. Hence, monostable or as it is sometimes referred to as a one-shot. The circuit maintains this single stable state except when a triggering pulse is applied. Then the state changes for a predetermined length of time set by the RC time constant of the AC coupled part of the signal path. In the bistable multivibrator both coupling paths are DC coupled and thus the circuit has two different stable states and uses no capacitors. The bistable multivibrator is also called a flip-flop, with either of two DC stable states.
The Astable Multivibrator
Objectives:
The objective of this first experiment is to build an astable multivibrator. Two identical resistance-capacitance networks determine the frequency at which oscillation will occur. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 1.
Materials:
ADALM1000 hardware module
Solder-less breadboard and Jumper wire kit
2 - 470 Ω Resistors
2 - 20 KΩ Resistors
2 - small signal NPN transistors (2N3904)
1 - Red LED
1 - Green LED
2 - 47 uF Capacitors
Directions:
Construct the circuit as shown in figure 1 on your solder-less breadboard. Note: there is no input from the M1000 just the power supply. The first inverting amplifier stage consists of Q1 with R1 and the Red LED serving as the output load. The second inverting amplifier stage consists of Q2 with R2 and the Green LED serving as the load. C1 couples the output of the first stage at the collector of Q1 to the input of the second stage at the base of Q2. Similarly, C2 couples the output of the second stage at the collector of Q2 back to the input of the first stage at the base of Q1.
Figure 1, Astable Multivibrator
Procedure:
Connect the +5 V power supply only after you have completely built and checked the circuit. The red and green LEDs should alternately blink on and off at about a 1 second interval. You can also use the scope channels in high impedance mode to monitor the output waveforms (Q and Qbar).
The frequency of oscillation is very slow due to the large values of capacitors C1 and C2. Replace C1 and C2 with 0.1 uF capacitors. The circuit should oscillate much faster now such that both LEDs seem to be on at the same time. Using the scope channels you can now measure the frequency and period of the output waveforms.
Questions:
What would be the effect of decreasing or increasing the values of both capacitors?
What would be the effect of decreasing C1 and increasing C2 but keeping the total of C1+C2 the same?
The Monostable Multivibrator
Objectives:
The objective of this second experiment is to build an monostable multivibrator. One resistance-capacitance network determines the duration of the one-shot output. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 2.
Materials:
ADALM1000 hardware module
Solder-less breadboard and Jumper wire kit
2 - 470 Ω Resistors
1 - 1 KΩ Resistor
1 - 20 KΩ Resistor
1 - 47 KΩ Resistor
1 - small signal diode (1N914)
2 - small signal NPN transistors (2N3904)
1 - Red LED
1 - Green LED
1 - 47 uF Capacitor
Directions:
Construct the circuit as shown in figure 2 on your solder-less breadboard. Starting with the circuit from experiment 1, remove one of the 20 KΩ resistors (old R3) and replace capacitor C1 with a 47 KΩ resistor (new R3). Add diode D1 and resistor R5 as shown to the base of Q2. Be sure to replace C2 with the original 47 uF capacitor.
Figure 2, Monostable Multivibrator
Procedure:
Connect the +5 V power supply only after you have completely built and checked the circuit. The red LED should be lit and the green LED should be dark. With a jumper wire, momentarily touch the trigger input (end of R5) to +5 V and immediately let go. The red LED should go out and the green LED come on for about a second and then go back to the stable state with the red on and green off. Try this a few times.
The width of the one-shot pulse is very long due to the large value of capacitor C2. Replace C2 with 0.1 uF capacitor. The output pulse should be much shorter now such that the green LED does not seem to flash on. Connect the channel A generator output to the trigger input. Configure channel A to force voltage with a Min of 0 V and a Max of 5V. Set the shape to a square wave, the frequency to 100 Hz and the duty cycle to 10% ( i.e. the pulse is high for 1/10 of the period). Use scope channel B to measure the width of the output Q and Qbar waveforms.
Questions:
What would be the effect of decreasing or increasing the value of capacitor C2?
The Bistable Multivibrator ( or flip-flop )
Objectives:
The objective of this third experiment is to build an bistable multivibrator. The amplifying devices (transistors) are connected in a common-emitter configuration, as shown in figure 3.
Materials:
ADALM1000 hardware module
Solder-less breadboard and Jumper wire kit\
2 - 470 Ω Resistors
2 - 1 KΩ Resistors
2 - 47 KΩ Resistors
2 - small signal NPN transistors (2N3904)
2 - small signal diodes (1N914)\
1 - Red LED
1 - Green LED
Directions:
Construct the bistable circuit as shown in figure 3 on your solder-less breadboard.
Figure 3, Bistable Multivibrator
Procedure:
Connect the +5 V power supply only after you have completely built and checked the circuit. Either the red LED should be lit with the green LED dark or the green LED should be lit with the red LED dark. With a length of wire, momentarily touch the either the SET or RESET input (end of R5 or R6) to +5 V and immediately let go. The LEDs should change state or toggle back and forth depending which input is touched to +5 V. Try this a few times.
Now connect the CA-OUT and CB-OUT of the M1000 to the SET and RESET inputs. With both channels configured in the Split I/O mode with the Termination to GND. Set their Min values to 0 mA and their Max values to 100 mA ( 0 to 5 V swing ). Set both channels frequency to 100 Hz, square wave shape. Set both duty-cycles to 10 % ( i.e. the pulse is high for 1/10 of the period). Set channels Phase to 0 and channel B to 180 degrees. The pulses should not overlap and be equally spaced from each other. Observe the relative timing and frequency of the waveforms at the Q and Qbar outputs.
Questions:
D-Type Flip-Flop
Objectives:
The objective of this fourth experiment is to use the bistable or set - reset flip-flop from experiment 3 to build what is known as a D-Type flip-flop.
Materials:
ADALM1000 hardware Module
Solder-less breadboard and Jumper wire kit
3 - 1 KΩ resistors
1 - 100 KΩ resistor
2 - 200 KΩ resistors
2 - 47 KΩ resistors
3 - small signal NPN transistors (2N3904)
2 - small signal diodes (1N914)
2 - 39 pF capacitors
2 - 100 pF capacitors
Directions:
Construct the D type flip-flop circuit as shown in figure 4 on your solder-less breadboard. Note that the polarity of the two diodes is reversed compared to figure 3. Because this experiment will be done at much higher frequencies, the LEDs have been removed and simple 1 KΩ load resistors are used.
Switching between the two flip-flop states is achieved by applying the D (data) signal and a single clock pulse which, depending on the state of the D input with respect to the current state will, cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative or falling edge of the clock pulse. The true D signal and complement DB signal ( output of Q3, R7 inverting stage ) are used to bias diodes D1 and D2 to steer the clock pulse to the correct base, the equivalent of the SET and RESET inputs in figure 3.
To illustrate how the circuit operates we will assume the circuit is in one of its two stable states with the QB output low ( collector voltage of Q1 at 0 V ), and the Q output high ( collector voltage of Q2 high at 5 V ). With the D input low ( DB high ) D1 has a low voltage on its cathode via R6 and a high voltage ( VBE of on transistor Q1 ) on its anode via R4, making it forward biased. D2 has a high voltage ( from DB ) on its cathode via R5 and a low voltage on its anode via R3 ( VBE of off transistor Q2 ), making it reverse biased.
A negative going pulse on the Clock input, coupled through C1 and C2, is steered to the base of Q1 since D1 is forward biased, but blocked from the base of Q2 by reverse biased D2. Q1 is turned off and Q2 is turned on by the cross coupled connection through the parallel combination of C3 and R3. This happens very quickly because of the positive feedback effect we saw earlier in the simple bistable multivibrator. The circuit is now in the other stable state with the Q output high and the QB output low. The circuit will remain in that state until the D input becomes high and after another negative going clock pulse arrives.
Figure 4 D type flip-flop
Hardware setup:
The channel A generator output CA-OUT should be connected to the input marked D in figure 4. The channel B generator output CB-OUT should be connected to the Clock input. The channel A input CA-IN should be connected to Q output. The channel B input CB-IN should be connected to the Qbar output of the flip-flop in figure 4.
With both channels configured in the Split I/O mode with the Termination to GND. Set their Min values to 0 mA and their Max values to 100 mA ( 0 to 5 V swing ). Set channel A frequency to 100 Hz, and square wave shape. Set channel B frequency to 200 Hz, and square wave shape. Set both duty-cycles to 50 % ( i.e. the pulse is high for 1/2 of the period). Set the phase of CA to 45 degrees. Be sure to configure the AWG outputs to operate synchronously.
Procedure:
Connect the +5 V power supply and enable the generator outputs only after you have completely built and checked the circuit. You should observe a square wave on the Q output which is aligned with the falling edge of the Clock input signal. Change the phase of Channel A ( D input signal ) while observing this alignment. Does this change as the phase of the D input change? Move the channel CB-IN scope input to the D input. You should see a similar square wave signal but ahead in time with respect to the Q output. In other words the Q output is delayed until the falling edge of the Clock signal.
Questions:
What is the purpose of capacitors C3 and C4?
What if the highest clock frequency at which the circuit continues to function? What limits this maximum frequency?
The capacitor coupling ( C1 and C2 AC coupling ) of the Clock input relies on the rise and fall time ( dV/dT ) of the input pulse to switch the state of the flip-flop. Place a capacitor from the CB-OUT pin to GND. This capacitor along with the 50 ohm termination resistor will determine the rise / fall time of the input pulse. Calculate different RC combinations to determine the minimum rise / fall times that will trigger the flip-flop circuit.
Divide by 2 Flip-Flop
Objectives:
The objective of this fifth experiment is to modify the D-type flip-flop from experiment 4 to build a circuit that divides the frequency of an input signal by 2.
Materials:
ADALM1000 hardware module
Solder-less breadboard and Jumper wire kit
2 - 1 KΩ resistors
2 - 200 KΩ resistors
2 - 47 KΩ resistors
2 - small signal NPN transistors (2N3904)
2 - small signal diodes (1N914)
2 - 39 pF capacitors
2 - 100 pF capacitors
Directions:
Modify the D-type flip-flop from experiment four to construct the divide by 2 circuit as shown in figure 5 on your solder-less breadboard.
Switching between the two states is achieved by applying a single clock pulse which in turn will cause the “ON” transistor to turn “OFF” and the “OFF” transistor to turn “ON” on the negative or falling edge of the clock pulse. The circuit will switch sequentially by applying a pulse to each base in turn and this is achieved from a single input clock pulse using biasing the two diodes to steer the pulse to the correct base based on the current state of the flip-flop.
To illustrate how the circuit operates we will assume the circuit is in one of its two stable states with the collector voltage of Q1 low (0 V), and that of Q2 high (5 V). D1 has a low voltage on its cathode via R6 and a high voltage ( VBE of on transistor Q1 ) on its anode via R4, making it forward biased. D2 has a high voltage on its cathode via R5 and a low voltage on its anode via R3 ( VBE of off transistor Q2 ), making it reverse biased.
An external negative going pulse, coupled through C1 and C2, is steered to the base of Q1 since D1 is forward biased, but blocked from the base of Q2 by reverse biased D2. Q1 is turned off and Q2 is turned on by the cross coupled connection through the parallel combination of C3 and R3. This happens very quickly because of the positive feedback effect we saw earlier in the simple bistable multivibrator. The circuit is now in its second stable state and waits for another negative going clock pulse.
Since the collector voltage of Q2 , the Q output node, changes state for every clock pulse, there is one pulse appearing at the output for every two clock input pulses. It can therefore be used as a divide by two circuit.
Figure 5 Divide by 2 circuit
Hardware setup:
The channel A output should be connected to the input marked Clock in figure 5. The scope input channel B should be connected to the Q output of the flip-flop in figure 5. The CA-V should be configured as a square wave with a 5 V Max and 0 V Min ( 0 - 5V swing ). Set the frequency to 500 Hz.
Procedure:
Connect the +5 V power supply and enable generator output only after you have completely built and checked the circuit. You should observe a square wave on the Q output which is one half the frequency of the CA-V signal. Move the channel B scope input to the QB output. You should see a similar square wave signal but inverted with respect to the Q output.
Questions:
What if the highest clock frequency at which the circuit continues to function? What limits this maximum frequency?
Reverse the polarity ( direction ) of the two steering diodes, D1 and D2. What is the effect on the relative timing of the Q and QB outputs with respect to the input clock signal? Explain why it has changed.
For further reading:
http://en.wikipedia.org/wiki/Multivibrator
http://www.wisc-online.com/objects/ViewObject.aspx?ID=DIG5303
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