Question

a. Create NAND gate using only transistors.

b. Using the circuit block Create NAND gate using only transistors , attach additional components to the circuit to have the output of AND

gate.

c. Explain both circuits

d. Out of CB, CC and CE which circuit will you use to amplify the sound output of your computer and why. Explain.

Answer 1

a)The NAND gate can be construted by using the transistors as TTL logic family as shown below

This constructed by using as special type of transistor as multiple emmiter transistor i.e., two emmitter transistor.

b) By placing the extra transistor and resistor  in between the T₃ and T₄ the above circuit eill behaves like a AND gate.

c)operation of a) circuit

(i)when the two inputs are high,the transistor T₁ has no emitter current because emitter-base junction is reverse biased.Hence it is OFF.The base-collector junction of T₁ is forward biased therefore it supply the base current to T₂.Hence T₂ is tured ON.Now its collector potential goes low which cuts OFF T₃.At the same time T₂ feeds the base to T₄ i.e., T₄ is turned ON.So, T₄ is in saturation whose output is in low state

Hence output is "0"

(ii)Whenthe two inputs are low,The transistor T₁ will have base -emitter current T₁ is ON with collector viltage is low i.e., placing the transisor T₂ to ground.So, thre transistor T₂ is turned OFF.Consequently T₃ is ON and T₄ is OFF.Now the output is high "1"

Input A	Intput B	Output
0	0	1
0	1	0
1	0	0
1	1	0

Hence the circuit is behaves like a NAND gate

operation of b) circuit  

i)When the two inputs are high the output is high.

ii)when the two inputs are low the output is low.

Input A	Input B	Output
0	0	0
0	1	0
1	0	0
1	1	1

From above two cases the circuit will behaves like a AND gate .

d)The CB -configuration of the transistor will behaves like a amplifier.

The above circuit will amplifies the weak signal to the output.

A small change in signal voltage produces an appreciable change in emitter current because the input circuit has low resistance.Now ,due to the transistor action the change in emitter current causes almost the same change in collectoe current.when the collector current flows through the load resistance R_L,a large voltage is developed across it.In this way a weak signal applied in the input signal appears in the amplified form.

Let a small voltage applied between the base and emiiter junction causes a relatively large emitter-current change.