# Logic Function with transistors

Discussion in 'Homework Help' started by AD633, Jun 23, 2013.

Jun 22, 2013
96
1
Hi,

Consider the circuit of Figure 3 where the transistors operate in a regime switching (cut / saturation).

Complete the table to the side and say what logical function is implemented by the circuit. Justify the values​​, presenting the state of conduction of each transistor.

When we have V1 = V2 = +5 V and +5 V.

$T1:
VCE1=Vcc-Rc1*Ic
VCE1=10-(5*10^3)*Ic$

I don't know Ic and i can not determine Ib because i also dont know the value of the base resistance Rb

$5=(Rb??)*Ib+Vbe$

Is there any other way for determing Ic?

Thanks

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2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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But why you need Ib and Ic current? You have been told that BJT work only in two regions of operation. The cut-off region and saturation region. So if for example V1 = +5V, T1 is in saturation region Vce1 = 0V And T3 will be OFF (cut-off) regardless T2 state.

Jun 22, 2013
96
1
So its fairly simple to determine what is the logic function implemented by the circuit:

V1 = V2 = +5 V and +5 V

VCE1=0 V and VCE2=0 V

V1 =+5 V and V2= 0 V

VCE1=0V and VCE2=+VCC

V1 =0 V and V2=+5 V

VCE1=+VCC and VCE2=0 V

V1 =0 V and V2=0 V

VCE1=+5V and VCE2=+5 V

So i think that the logic function implemented by the circuit is a NOT.

4. ### Brownout Well-Known Member

Jan 10, 2012
2,375
998
Good so far.

That would be impossible

Not quite.

1) Why would a NOT function have two inputs?
2) Why are you ignoring t3?

Jun 22, 2013
96
1

Yes it should be, VCE1=0V and VCE2= 0V

I did not understand this.Are you saying that it should be

VCE1=+VCC-(Rc1)*(Ic1sat) and VCE2=0 V

Yes it would not make much sense

I forgot about the NMOS.In a NMOS transistor we need to have a positive Vgs potential so that we get a high sinal in the output right?

The logic function is a NAND?

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,019
1,123
Look at the circuit diagram and notice that if T1 is in cut-off And T2 is in saturation, we have two Rc resistors connected in series.
Vc2 = 0V and Vce1 = ??

No, the NMOS work just like a inverter. Vgs = 5V---> Vds --->0V