The Buffer Gate - Introduction &Design

 

The Buffer:

The first basic gate is a buffer. Output is the same as the input. 

Learning Objectives:

• Introducing buffer implementation using

• Switches

• Diodes

The logic symbol is the same as that of a NOT gate except the bubble. 

Logical Expression:

A = Y

The Buffer
Input A	Output Y
0	0
1	1

Logical Buffer (Explain with the help of switches)

The switch model

In practical life, we never use a switch as a buffer. The purpose of a switch model is to explain easily. Just like a switch, its purpose is to transfer the input to the output. There are so many different ways to implement the buffer logic. We will limit our study to a very basic buffer circuit. 

In digital circuit design, there are various ways to implement the buffer. 

Two inverters in series can act as a buffer:

Simple and easy task. Two inverters in series will produce the same logic as that of input. In this way the input inverses two times and the output is the same as that of the input.

Buffer with the help of an AND gate:

Consider a two-input AND gate. Connect both of its input terminals. It works as a buffer. Have a look at the schematic diagram.

Buffer with the help of an OR gate:

Consider a two-input OR gate. Connect both of its terminals. It will act as a buffer. 

Note: there is a difference between input wave and output wave amplitude. This is because every practical circuit experiences a voltage drop. The drop is due to the internal circuitry of the logic gates.

Three different ways to implement Buffer Logic

Implementation Using CMOS Logic

Two back to back inverters work as a buffer. 

The CMOS Buffer Circuit

The XNOR Gate

The XNOR Gate:

The logic symbol is the same as the XOR gate with an inversion bubble placed at the output side. The exclusive NOR gate produces an inverted output as that of the XOR gate.

Learning Objectives:

• Introducing XNOR gate implementation using

• Switches

• Diodes

• BJT

Working:

Case 1: 

Input A = 0

Input B = 0

Output = 1

Case 2:

Input A = 0

Input B = 1

Output = 0

Case 3:

Input A = 1

Input B = 0

Output = 0

Case 4:

Input A = 1

Input B = 1

Output = 1

2 Input XNOR Gate
Input A	Input B	Output
0	0	1
1	0	0
0	1	0
1	1	1

Logical Expression:

Y = A + B

Logical XNOR Gate (Explain with the help of switches)

The XNOR switch model contains 4 switches. Switch A and its complement switch AC. Similarly, there is a switch B and its complement switch BC. Output goes high when both inputs are either zero or high. The switch circuit fulfils both conditions. When switch A and switch B are closed (logic 1), output goes high. Similarly, when switch A and switch B are opened (logic 0) the complement switches AC and BC are logic high. Output goes high in this case as well.

Implementation Using Diode Logic

The XNOR gate diode (DTL) circuit was not an easy task. I tried many different ways but I failed. Finally, I draw a schematic that contains a transistor at the end of the bridge circuit. It performs an inversion operation. The whole circuit is similar to the XOR diode circuit except for the last transistor. 

Case 1:

Look at the schematic in case 1. Both switches are open. All diodes remain turned off. The transistor is also turned off. Output (LED is turned on) is high.

Case 2:

In this case, switch S1 is connected to a 5V source while S2 remains connected to the ground. S1 is connected to the diode D1, and D1 is connected to the transistor Q1. It turns on Q1. No current flows from the LED and hence the output is low.

Case 3:

Same as case 2.

Case 4:

Look at the schematic in case 4. Switches S1 and S2 are connected to 5V sources. S1 turns on D1 and S2 turns on D2. Look at node 2. There is no potential difference at this node and hence no current will flow from this path. No current will flow to the base of Q1 and transistor Q1 will remain turned off. Output goes high.

Implementation Using Transistor Logic

It is surprisingly easy to design a BJT based XNOR gate. It only contains two BJT transistors. Collectors are connected. Bases are connected to the 5V source via switches S1 and S2. The emitter of Q1 is connected to S2 and the emitter of Q2 is connected to S1. The configuration is the same as that of the XOR gate. The only difference is the third Transistor which performs inversion operation (in the case of the XOR gate). 

Case 1:

Look at the schematic, both switches S1 and S2 are open. Both transistors are off. No current will flow from the transistors. The output will be high.

Case 2:

In this case, switch S1 is connected to a 5V source. The base of Q1 is connected to S1 and the emitter of Q1 is connected to S2 which is grounded. Transistor Q1 is on and all the current flows through Q1. Output remains low.

Case 3: 

Same as case 2.

Case 4:

Look at the schematic in case 4. Both switches S1 and S2 are connected to 5V sources. The base of Q1 is connected to V1 and the emitter of Q1 is connected to V2. So, the is no potential difference between base and emitter and hence

VBE = 0

Similarly, this is true for Q2. Both transistors are off. And hence output goes high.

The XOR Gate

 The logic symbol is the same as the XOR gate with an inversion bubble placed at the output side.

Learning Objectives:

• Introducing AND gate implementation using

• Switches

• Diodes

• BJT

Working:

Case 1: 

Input A = 0

Input B = 0

Output = 0

Case 2:

Input A = 0

Input B = 1

Output = 1

Case 3:

Input A = 1

Input B = 0

Output = 1

Case 4:

Input A = 1

Input B = 1

Output = 0

Logical Expression

Y = A ⊕ B

2 Input XOR Gate
Input A	Input B	Output
0	0	0
1	0	1
0	1	1
1	1	0

Logical XOR Gate (Explain with the help of switches)

Implementation Using Diode Logic

XOR is not among the basic logic gates. There are many different ways to implement XOR logic with the help of diodes. I tried the circuit below. The circuit is for learning purposes. The diode logic is no longer used these days.

Case 1:

Both the inputs are connected to the ground. All diodes are off. The output of the circuit is zero (logic 0).

Case 2:

In this case, look at the schematic, switch S1 is closed. Diode D1, LED, D2, D4 turns ON. 

Case 3: 

Look at the schematic in case 3. Switch S2 is closed. Diodes D2, LED and D4 turn ON. 

Case 4:

Look at the schematic in case 4. Switch S1 and S2 both are closed (connected to the 5V sources). Carefully looking at all the junctions, there is no potential difference between these junctions ideally (there is no potential drop across ideal diodes). And hence no current flows through any diode.

Implementation Using Transistor Logic

XOR is a little bit difficult to implement with the help of BJT. There are many other ways to implement XOR logic. I come up with this simple circuit that consists of three BJT.

Case 1:

Look at the schematic in case 1. Both switches are tied to the ground and hence Q1, Q2 and Q3 transistors are off. Output is zero (logic low).

Case 2:

Look at the schematic, switch 1 is connected to a 5V source. Closely looked at Q1. The base of the Q1 transistor is connected to a 5V source. The collector is powered by a 12V source and the emitter is connected to the ground. It is a common emitter configuration. Transistor Q1 is ON while Q2 is OFF. The output of the Q1 transistor is connected to the base of Q3. The positive voltage at the base will turn ON Q3 and hence output goes high. 

Case 3:

Same as case 2.

Case 4:

Look at the schematic in case 4. The base of the transistor Q1 is connected to a 5V source and the emitter is also connected to a 5V source. To operate a transistor VBE = VB - VE ≥ 0.7V. 

In our case VBE = 0. Since base and emitter are at the same potential. So, transistor Q1 is off. Similarly, this is true for Q2. Both transistors (Q1 and Q2) are off. There is no voltage at the base of Q3. So, the output is also low. 

The NOR Gate

 The OR Gate - Truth Table, Timing Diagram, Switch Model:

The logic symbol is the same as the OR gate with an inversion bubble placed at the output side. Its function is NOT-OR. It has a similar truth table to that of a NAND gate. Its output is high only when both of the inputs are low. 

Learning Objectives:

• Introducing NOR gate implementation using

• Switches

• Diodes

• BJT

Working:

Case 1: 

Input A = 0

Input B = 0

Output = 1

Case 2:

Input A = 0

Input B = 1

Output = 1

Case 3:

Input A = 1

Input B = 0

Output = 0

Case 4:

Input A = 1

Input B = 1

Output = 0

In figure 1 the waveforms or timing diagram of the NOR gate is also given. The input and output signals are represented by the pulses (a pulse of square wave). There are two inputs A and B and output is Y as shown. The output pulse falls to 0V when any one of the two inputs are high.

Figure 1: NOR gate - schematic symbol, truth table, logical equation and waveforms

Figure 2: Simple circuit simulation results

Truth Table Of 2-Input NOR Gate:

2 Input NOR Gate
Input A	Input B	Output
0	0	1
1	0	0
0	1	0
1	1	0

Logical Expression:

Y = (A + B)C

Logical NOR Gate (Explain with the help of switches) | NOR Gate Switch Logic:

Figure 3: Switch Model of NOR gate

In the above figure, there is a switch model of a NOR gate. It is easy to understand the operation of the gate with the help of switches. The inputs are represented by two switches A and B. The output is represented by an LED. The LED turns ON only when both of the switches are off (open). The only path for the current to flow is from the LED. If anyone switch turns ON, the current will flow from the least resistive path.

Implementation Using Diode Logic

Figure 4: Diode circuit of NOR gate

Case 1:

Both switches are off (opened), and hence both diodes are off. Current flows from V3 to LED and ground.

Case 2:

In case 2, look at the schematic. Switch S1 is ON while S2 is off. The positive terminal of the diode is connected to a 5V source and hence turns on. On the negative terminal of D1, there is a positive potential because of V3. So, there is no or very little current will flow.

Case 3:

Same as case 2.

Case 4:

Same as case 2 and case 3.

Implementation Using Transistor Logic

NOR gate is easily implemented with the help of two BJT transistors. The transistors are connected in differential pair form. It is a common emitter configuration. The output is taken from the collector while the input is at base.

Figure 5: BJT circuit of NOR gate

Case 1:

Look at the schematic diagram in case 1. Both switches are off. Both transistors are off. The only path available for the current is through the LED. And Hence LED glows. 

Case 2:

In case 2, S1 is on and hence Q1 is ON. The current will flow from Q1 to the ground. No current will flow through the LED.

Case 3:

Same as case 2.

Case 4:

Same as case 2 and case 3.