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Activity 24. BJT Multivibrators

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:

ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
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. The green boxes indicate connections to the ADALM2000. Note: there is no input from the ADALM2000 board 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:

Turn on the Vp 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 to monitor the output waveforms (Q and Qbar).

Figure 2, Astable Multivibrator Breadboard Circuit

The frequency of oscillation is very slow due to the large values of capacitors C₁ and C₂. Replace C₁ and C₂ with 0.1uF 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 should now measure the frequency and period of the output waveforms.

Figure 3, Astable Multivibrator interval at 47uF capacitor

Figure 4, Astable Multivibrator interval at 0.1uF capacitor

Questions:

1. What are the two most important components in the multivibrator circuit shown in figure 1?
2. What would be the effect of increasing or decreasing the value of only one capacitor?
3. What would be the effect of increasing or decreasing the value of both capacitors?

Add more questions here:

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:

ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
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. The green boxes indicate connections to the ADALM2000. Starting with the circuit from experiment 1, remove one of the 20K? resistors (old R₃) and replace capacitor C₁ with a 47K? 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 5, Monostable Multivibrator

Figure 6, Monostable Multivibrator Breadboard Circuit

Procedure:

Turn on the Vp 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 length of wire, momentarily touch the trigger input (end of R₅) to Vp 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.

Figure 6, Monostable Multivibrator Behavior on trigger

Questions:

Add questions here:

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:

ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
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 circuit as shown in figure 3 on your solder-less breadboard. The green boxes indicate connections to theADALM2000.

Figure 7, Bistable Multivibrator

Figure 8, Bistable Multivibrator Breadboard Circuit

Procedure:

Turn on the Vp 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 Vp and immediately let go. The LEDs should change state or toggle back and forth depending which input is touched to Vp. Try this a few times.

Figure 9, Bistable Multivibrator behavior triggering Set pin

Figure 10, Bistable Multivibrator behavior triggering Reset pin

Connect the SET and RESET inputs to two of the digital I/O pins on the ADALM2000 connector. Configure the pins as push-pull outputs. Used the static digital I/O screen to control the digital pins.

Questions:

Add Questions here:

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:

ADALM2000 Active Learning Module
Solder-less breadboard
Jumper wires
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 K? 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 ( V_BE 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 ( V_BE 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 11 D type flip-flop

Hardware setup:

The AWG1 output should be connected to the input marked Clock in figure 4. The AWG2 output should be connected to the D input. The first scope channel 1 input should also be connected to Clock input. The second input scope channel 2 should be connected to the Q output of the flip-flop in figure 4. Both the AWG1 and AWG2 should be configured as a square wave with a 5 V amplitude and 2.5 V offset ( 0 - 5V swing ). Set the frequency of AWG1 to 10 KHz and set the frequency of AWG2 to 5 KHz. Set the phase of AWG2 to 45 degrees. Be sure to configure the two AWG outputs to operate synchronously.

Figure 12 D type flip-flop breadboard circuit

Procedure:

Turn on the Vp power supply and enable the AWG 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 AWG2 ( D input signal ) while observing this alignment. Does this change as the phase of the D input change? Move the channel 1 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.

Figure 13: Plot of Q and Clock signal

Figure 14: Plot of Q and D 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 ( 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. Use the trapezoidal waveform option for AWG 1 and use the symmetry control to adjust the rise/fall time of the waveform. What is the slowest dV/dT that will change the state of the flip-flop?