LOGIC CIRCUITS WITH BJT
I. OBJECTIVES
a) Experimental determination of some logic functions for circuits from the RTL and DTL family.
b) Determination the VTC dependencies on the values of some resistors from the circuit.
II. COMPONENTS AND INSTRUMENTATION
You will use the experimental board containing 2N2222 npn type BJT, semiconductor diodes and
resistors. You will apply and measure ac and dc voltages, so you will use a dc regulated power supply, a
signal generator, a dual channel oscilloscope and a dc voltmeter.
III. PREPARATION
1.P. RTL INVERTER
The following logic convention is used:
- the high level of the voltage – “1” logic
- the low level of the voltage – “0” logic
1.1.P. THE LOGIC FUNCTION
•
Find the logic function implemented by the circuit from Fig. 1.
1.2.P. THE VTC DEPENDENCE ON THE RC
•
•
•
For T we consider: β=200, VBEon=0.6V, VCEsat=0.2V.
What does the VTC look like for the circuit from Fig. 1 with RB=22KΩ and RC=1kΩ?
What are the values of vA for which T is turned off? What about the saturation state?
What does the VTC look like if RC is 22kΩ? (RB=22kΩ)
1.3.P. THE VTC DEPENDENCE ON THE RB
•
•
What does the VTC from 1.2.P look like if RB=1kΩ (RC=1kΩ)?
What are the values of vY for which T is in off or saturation state?
2.P. THE NOR LOGIC CIRCUIT
2.1.P. NOR FROM THE RTL FAMILY
•
•
•
What is the operation table of the circuit from Fig. 2? vA,vB∈{0V,5V}
What are the states of TA and TB (off or saturation) for each combination of vA and vB?
What is the logic function of the circuit?
2.2.P. NOR WITH D AND T
•
•
Verify the logic function NOR for the circuit from Fig. 3 (space maximum DR circuit and logic
inverter)
Is the circuit from Fig. 3 more simple than the one from Fig. 2? From what point of view?
3.P. NAND FROM THE RTL FAMILY
•
Using 1KΩ and 22kΩ resistors and BJTs design a logic circuit (from RTL family) for the NAND logic
function with 2 inputs.
IV. EXPLORATIONS AND RESULTS
1.RTL INVERTER
1.1. THE LOGIC FUNCTION
Exploration
Build the circuit shown in Fig. 1.
• At the A input is applied a TTL signal with 1kHz frequency obtained from the signal generator.
• Using the calibrated oscilloscope in the Y-t mode, visualise vA(t) and vY(t).
Results
• vA(t), vY(t).
• The truth table in which A and Y are the input and output logic variables.
• What is the logic function of the circuit?
1.2. THE VTC DEPENDENCE ON THE RC
Exploration
Build the circuit shown in Fig. 1.
Vcc =5V
RC
Y
A
RB
T
vY
vA
Fig. 1. The RTL inverter
• vA(t)=5 sin(2π 1000t) [V] [Hz]
• Using the oscilloscope in the Y-X mode, visualise the VTC vY(vA), for the next cases:
a) RB=22kΩ
b) RB=22kΩ
RC=1kΩ
RC=22kΩ
Results
• Draw the VTC vY(vA) for the above cases.
• Mark the on, off and saturation regions of BJT on the plot.
1.3. THE VTC DEPENDENCE ON THE RB
Exploration
Build the circuit shown in Fig. 1.
• vA=5 sin (2π1000t)[V][Hz]
• Using of the oscilloscope you will visualise vY(vA) for
a) RB=22kΩ
RC=1kΩ
b) RB=1kΩ
RC=1kΩ
Results
• Draw the VTC vY(vA) for the two above cases.
• How does RB influence the range of values of vA for which BJT is off and in saturation?
2. THE NOR LOGIC CIRCUIT
2.1. NOR FROM THE RTL FAMILY
Exploration
Build the circuit shown in Fig. 2.
• vA, vB∈{0V;5V} in all possible combinations
• Measure vY with the dc voltmeter for all possible combinations of the two input voltages.
Results
• Electric table with vA,vB,vY, the off or saturation states of TA and TB for the 4 possible combinations of
vA and vB values from {0V;5V}
• Truth table with A, B logic inputs and Y logic output
• What is the logic function? Why?
Vcc =5V
1K
RC
Y
RB
A
22k
vA
B
TA
RB
TB
vY
22K
VB
Fig. 2. NOR from RTL family
2.2. NOR WITH D AND T
Vcc =5V
DA
RC
RB1
vA
vB
DB
1K
T
VY
22k
RB2
22k
Fig. 3. NOR from DTL family
Exploration
• In order to reduce the number of transistors and the complexity of the circuit we can use two diodes – Fig.
3. Build the circuit.
• The experiment is similar to the one from experiment 2.1.
Results
• Electric table with vA,vB,vy, for all four possible combinations of the values vA and vB from the set
{0V;5V}.
• Why do we need RB2?
• Truth table with A and B as logic input and Y as logic output
• How do you explain that the circuits from Fig. 2 and Fig. 3 make the same logic function?
3. NAND FROM THE RTL FAMILY
Exploration
• Build the circuit designed in the 3.P paragraph
• In the same way with the experiment 2.1 verify experimentally the logic function.
Results
• Truth table for your circuit.
REFERENCES
1. Oltean, G., Electronic Devices, Editura U.T. Pres, Cluj-Napoca, ISBN 973-662-220-7, 2006
2. Sedra, A. S., Smith, K. C., Microelectronic Circuits, Fifth Edition, Oxford University Press, ISBN: 0-19514252-7, 2004
3. http://www.bel.utcluj.ro/dce/didactic/ed/ed.htm
Fig. 4. Experimental assembly