Digital Circuit Design

Objective:

The objective of this experiment is to study three fundamental digital circuits using BJTs and diodes. These three circuits are:

  • BJT Inverter
  • Diode AND
  • BJT, Diode NAND

Introduction

To achieve digital logic levels in practical circuitry, two controllable stable states are required. The logic levels are referred to in the binary number system as 'zero' and 'one'. The two stable states are referred to as 'low' and 'high'. In the laboratory, the current switching properties of bipolar junction transistors and diodes can be used to develop analog circuits which form the basic building blocks required to implement digital logic processes. Three basic digital circuits will be built in this experiment.

Inverter Circuit

When the input signal applied to the circuit of Figure 1 is at a low voltage level, (less than 0.7 Volts) the base-emitter junction of the transistor is driven into cut-off. In this case, no current flows through R2, (IC=O). The output voltage will be high and equal to the DC supply voltage. When the input voltage becomes greater than 0.7 volts, the transistor is driven into saturation. The output voltage will be low and almost equal to the junction saturation voltage VCE,SAT (about 0.3 Volts). The transistor of this circuit functions as a switch and the circuit operates as an inverter. (Multisim Circuit, with Oscope)


Figure 1 - BJT Inverter Circuit

AND Gate Circuit

In the circuit of Figure 2, when both input sources A and B are at high enough voltage levels, (~ 5 Volts) diodes D1 and D2 are reverse biased and no current will flow through R1, (IR1 = 0). In this case, the output voltage level will be the same as the power supply voltage level (~ 5 Volts). However, when one or both inputs are at zero voltage levels, one or both diodes will be forward biased and a current will flow through R1. In this case, the output voltage level is low and almost equal to the diode voltage drop. (~ 0.7 Volts) (Multisim Circuit).


Figure 2 - Diode AND Circuit

NAND Gate Circuit

The NAND gate circuit of Figure 3 can be formed by combining the inverter circuit of Figure 1, and the AND gate circuit of Figure 2. In this circuit, when one or both inputs A and B are at low voltage level (compared to the power supply voltage level) either diode D1 or D2 or both will be forward biased. In such a case, the voltage drop across diode D1 or D2 will not be large enough to forward bias the base emitter junction and the transistor will be driven into cut-off. Hence no collector current will flow through R2 and the output voltage VOUTPUT is equal to the power supply voltage VCC. When both inputs A and B are at high voltage levels (5 Volts), both diodes will be reverse biased and the transistor will be in saturation. Therefore, the output voltage will be low. (Multisim Circuit)


Figure 3 - BJT - Diode NAND Circuit

Lab Work:

  1. Testing of Inverter (NOT) Circuit:
    Form the circuit of Figure 1, and construct the truth table experimentally. Apply a 100Hz triangle wave signal to the input and observe both the input and the output on the oscilloscope. Save both signals superimposed as part of your report.
  2. Testing of AND Gate Circuit:
    Form the circuit of Figure 2 and construct the truth table experimentally. Apply the output of the transistor switch circuit (NOT gate) of Figure 1 to the B input. Save both the input and output signals again superimposed for the following cases.
    1. Input terminal A connected to ground
    2. Input terminal A connected to +5 volts
  3. Testing of NAND Gate Circuit:
    Form the circuit of Figure 3 and construct the truth table experimentally. Apply 5 Volt peak to peak, zero DC offset triangle wave to the input A and obtain the graphs of the output waveform for the following cases.
    1. Input terminal B connected to ground
    2. Input terminal B connected to +5 volts

Results:

Send all your findings to the instructor by the end of the session through email as a MS Word attachment.