Logic Circuits with Bjt

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

· Show that the circuit from the Fig. 12.1 implements the logic function “NOT”.

· What does the schematic circuit with two resistors and one transistor look like in order to obtain the complementary logic function of the one above?

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.12.1 with RB=22KW and RC=1kW?

· What are the superior and inferior noise margins (NMH and NML) for RB=22KW and RC=1kW?

· 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 22kW? (RB=22kW)

1.3.P. THE VTC DEPENDENCE on THE RB

· What does the VTC from 1.2.P look like if RB=1kW (RC=1kW)?

· What are the values of vY for which T is in off or saturation state?

· Which one of the noise margins is modified and how for RB=1kW?

2.P. THE NOR LOGIC CIRCUIT

2.1.P. NOR FROM THE RTL FAMILY

· What is the operation table of the circuit from Fig 12.2.? vA,vBÎ{0V,5V}

· What are the states of TA and TB (off or saturation) for each combination of vA and vB?

· What voltage values correspond to the “0” and “1” logic levels at the input and output of the circuit?

· What is the logic function of the circuit?

2.2.P. NOR with D and T

· How can you verify the logic function NOR for the circuit from Fig 12.3.?

· Is the circuit from Fig 12.3. more simple than the one from Fig 12.2.? From what point of view?

3.P. NAND FROM THE RTL FAMILY

· Using 1KW and 22kW resistors and BJTs design a logic circuit (from RTL family) for the NAND logic function with 2 inputs.

· How you would experimentally check up the logic function of your circuit?

IV. Exploration and results

1.RTL inverter

1.1. THE LOGIC FUNCTION

Exploration

Build the circuit shown in Fig.12.1.

· 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 you will 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.12.1.

· vA(t)=5 sin(2p 1000t) [V] [Hz]

· Using the oscilloscope in the Y-X mode you will visualise the VTC vY(vA), connecting the X and Y probes of the oscilloscope with the points A and Y for the next cases:

a) RB=22kW b) RB=22kW

RC=1kW RC=22kW

Results

· Draw the VTC vY(vA) for the above cases.

· Mark the on, off and saturation regions of BJT on the same plot.

· What are the values of vA for which BJT is in off or respectively saturated state?

· How are these values influenced by the value of RC?

1.3. THE VTC DEPENDENCE ON THE RB

Exploration

Build the circuit shown in Fig.12.1.

· VA=5 sin (2p1000t)[V][Hz]

· Using of the oscilloscope you will visualise vY(vo) for

a) RB=22kW b) RB=1kW

RC=1kW RC=1kW

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?

· What are the values of vY for which the BJT is off and in saturation?

· What are the superior and inferior noise margins (NMH and NML) computed for the situations a) and b)?

· What is the role of RB (compared to the situation when RB=0)?

2. THE NOR LOGIC CIRCUIT

2.1. NOR FROM THE RTL FAMILY

Exploration

Build the circuit shown in Fig.12.2.

· vA, vBÎ{0V;5V} in all possible combinations

· You will measure vY with the dc voltmeter for all possible combinations of the two input voltages.

Results

· Operation 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?

2.2. NOR with D and T

Exploration

· In order to reduce the number of transistors and the complexity of the circuit we may use two diodes– Fig 12.3. Build the circuit.

· The experiment is similar to the one from 2.1.Exploration

Results

· Operation 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.12.2 and Fig.12.3 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 2.1 Exploration paragraph verify experimentally the logic function

Results

· Truth table for your circuit

· Congratulations for obtaining the desired logic function