Download Theoretical Background

Islamic University of Gaza
Faculty of Engineering
Electrical Engineering department
Experiment 2
Digital Electronics Lab (EELE 3121)
Eng. Mohammed S. Jouda
Eng. Amani S. abu reyala
BJT Inverter
Objectives


To be familiar with the operation of BJT Amplifier.
To determine VTC of the inverter.
Theoretical Background
1. Ideal Inverter Digital Gate
The ideal Inverter model is important because it gives a metric by which we can judge
the quality of actual implementation. Its VTC is shown in figure 1.1 and has the
following properties:
Infinite gain in the transition region, and gate threshold located in the middle of the
logic swing, with high and low margins equal to the half of the swing. The input and
output impedance of the ideal gate are infinity and zero, respectively.
2. Dynamic Behavior of Inverter Digital Gate
Figure1.2 illustrates the behavior of the inverter digital gate using BJT
There are three regions for the above voltage transfer characteristic
1. Cut-off region.
2. Forward Active region.
3. Saturation region.
BJT Inverter can be best expressed by its voltage transfer characteristic (VTC) or DC
transfer characteristic as shown in figure 1.3. That relates the output voltage to the
input one.
If:
 Vi = Vol, Vo = Voh = Vcc (VTC) or DC Transfer Characteristic
The transistor is OFF.

Vi = Vil
The transistor Begins to turn on.

Vil < Vi < Vih
The transistor is in forward active region and operates as Amplifier.

Vi = Voh
The transistor will be deep is saturation, Vo = Vce(sat).
 A measure of sensitivity to noise is called Noise Margin (NM) which can be
expressed by:
Nml = Vil – Vol.
Nmh = Voh – Vih.
 Logic Swing: Ls = Voh – Vol
 We can calculate the transition width using the following expression
Tw = Vih - Vil.
 Another point of interest of the VTC is the gate or switching threshold voltage
Vm that defines as Vm = F(Vm).
Vm can also be found graphically at the intersection og the VTC curve and the
line given by Vout = Vin as shown in Figure 1.3
 For an AC input, the propagation delay can be defined as:
Tphl: the response from a low to high transition.
Tplh: the response from a high to low
transition.
Tp: Overall propagation delay:
Tp = (Tphl + Tplh)/2.
Tr: Rising Time.
Tf: Falling Time.
Procedure
Part A:
1) Write the circuit shown in Figure 1.5:
2) Vary the input Voltage according to the table 1.1
VI
Vo
0.1
Vil = ……..
Vih = …….
Vol = …….
Voh = ……
0.2
0.3
Vil
1
1.5
2
2.5
3
Tw = …….
Nmh = …..
Nml = …...
Ls = ……..
3) Draw the relation between Vo & Vin (Using Drawing paper)
3.5
Vih
Part B:
1) Write the circuit shown in figure 1.6:
2) Apply a square wave input (F = 1K HZ, Vp = 5V)
3) Using the Oscilloscope draw Vo&Vin in same paper and same scale.
4) From the graph measure:
Tplh = …………
Tphl = …………
Tp = …………...
Tr = …………....
Tf = ……………