This document summarizes the operation of various logic gates using NPN BJTs in LtSpice. It describes the inputs, outputs and operating points for: 1) A NOT gate with 0V and 5V inputs. 2) An AND gate with various combinations of 0V and 5V inputs. 3) An OR gate with various combinations of 0V and 5V inputs. 4) A NOR gate with 0V and 5V inputs on both transistors. For each configuration it analyzes whether the transistors are on or off based on the 0.7V threshold, and whether this results in output or not.

1. Gate using BJT
NPN-BJT
Using LtSpice
1)NOT Gate:
a) for input 0V
Output:
--- Operating Point ---
V(n001): 0.698264 voltage
V(n004): 6.98364e-009 voltage
V(n003): 0 voltage
V(n002): 6 voltage
Ic(Q1): 6.98464e-013 device_current
Ib(Q1): -6.98364e-013 device_current
Ie(Q1): -1.00007e-016 device_current
I(D1): 0.00530198 device_current
I(R2): 0.00530174 device_current
I(R1): -6.98364e-013 device_current
I(V2): 6.98364e-013 device_current
I(V1): -0.00530174 device_current

2. As the voltage applied is 0V at the input therefore the npn transistor don’t works
or it becomes open as it requires 0.7V to work thus the other 6V source is
passed to the led and some output is obtained as it glows and voltage drop is
observed.
b) For input 5V
Output:
--- Operating Point ---
V(n001): 0.0759633 voltage
V(n004): 0.823145 voltage
V(n003): 5 voltage
V(n002): 6 voltage
Ic(Q1): 0.00592406 device_current
Ib(Q1): 0.000417686 device_current
Ie(Q1): -0.00634174 device_current
I(D1): 2.5454e-013 device_current
I(R2): 0.00592404 device_current
I(R1): 0.000417685 device_current
I(V2): -0.000417685 device_current
I(V1): -0.00592404 device_current

3. As the voltage applied is 5V at the input therefore the npn transistor works or it
becomes short as it requires only 0.7V to work thus the other 6V source is
passed to the transistor and not to the led and no output is obtained as led does
not glows.
2) AND Gate:
a) for V2 and V3 is 0V
Output
--- Operating Point ---
V(n001): 9 voltage
V(n002): 9.00015e-008 voltage
V(n004): 4.97559e-005 voltage
V(n006): 4.99481e-013 voltage
V(n007): 9.03264e-016 voltage

4. V(n003): 0 voltage
V(n005): 0 voltage
Ic(Q2): 5.01403e-017 device_current
Ib(Q2): -4.99481e-017 device_current
Ie(Q2): -1.92184e-019 device_current
Ic(Q1): 9.0002e-012 device_current
Ib(Q1): -9.00015e-012 device_current
Ie(Q1): -5.01403e-017 device_current
I(D1): 1.25187e-027 device_current
I(R3): 1.92184e-019 device_current
I(R2): -4.99481e-017 device_current
I(R1): -9.00015e-012 device_current
I(V3): 4.99481e-017 device_current
I(V2): 9.00015e-012 device_current
I(V1): -9.0002e-012 device_current
As the voltage V2 and V3 is 0V, thus the both transistor don’t work as the npn
transistor requires 0.7V, so no output is obtained.
b) for V2 is 5V and V3 is 0V
Output

5. --- Operating Point ---
V(n001): 9 voltage
V(n002): 5 voltage
V(n004): 4.72315 voltage
V(n006): 4.72325e-008 voltage
V(n007): 4.70223e-013 voltage
V(n003): 5 voltage
V(n005): 0 voltage
Ic(Q2): 4.72335e-012 device_current
Ib(Q2): -4.72325e-012 device_current
Ie(Q2): -1.00047e-016 device_current
Ic(Q1): 8.45206e-012 device_current
Ib(Q1): -3.67873e-012 device_current
Ie(Q1): -4.77333e-012 device_current
I(D1): 6.52022e-025 device_current
I(R3): 1.00047e-016 device_current
I(R2): -4.72325e-012 device_current
I(R1): -3.67923e-012 device_current
I(V3): 4.72325e-012 device_current
I(V2): 3.67923e-012 device_current
I(V1): -8.40258e-012 device_current
As the voltage V2 is 5V and V3 is 0V, thus the transistor with V2 works or it
act as short as it is greater than the 0.7V but the transistor with V3 don’t works
as the npn transistor requires 0.7V but it is zero so it is open but overall no
output is obtained as both should work together for the output.
c) for V2 is 0V and V3 is 5V

6. Output
--- Operating Point ---
V(n001): 9 voltage
V(n002): 9.63518e-008 voltage
V(n004): 0.635083 voltage
V(n006): 1.3825 voltage
V(n007): 0.617155 voltage
V(n003): 0 voltage
V(n005): 5 voltage
Ic(Q2): -6.35083e-013 device_current
Ib(Q2): 0.00036175 device_current
Ie(Q2): -0.00036175 device_current
Ic(Q1): 9.0001e-012 device_current
Ib(Q1): -9.63518e-012 device_current
Ie(Q1): 6.35084e-013 device_current
I(D1): 0.00023044 device_current
I(R3): 0.00013131 device_current
I(R2): 0.00036175 device_current
I(R1): -9.63518e-012 device_current
I(V3): -0.00036175 device_current
I(V2): 9.63518e-012 device_current
I(V1): -9.0001e-012 device_current

7. As the voltage V2 is 0V and V3 is 5V, thus the transistor with V3 works or it
act as short as it is greater than the 0.7V but the transistor with V2 don’t works
as the npn transistor requires 0.7V but it is zero so it is open but overall no
output is obtained as both should work together for the output.
d) For V2 and V3 is 5V
Output
--- Operating Point ---
V(n001): 9 voltage
V(n002): 1.80559 voltage
V(n004): 0.941759 voltage
V(n006): 1.60923 voltage
V(n007): 0.745116 voltage
V(n003): 5 voltage
V(n005): 5 voltage
Ic(Q2): 0.0322636 device_current
Ib(Q2): 0.000339077 device_current
Ie(Q2): -0.0326027 device_current
Ic(Q1): 0.0319441 device_current

8. Ib(Q1): 0.000319441 device_current
Ie(Q1): -0.0322636 device_current
I(D1): 0.0324441 device_current
I(R3): 0.000158535 device_current
I(R2): 0.000339077 device_current
I(R1): 0.000319441 device_current
I(V3): -0.000339077 device_current
I(V2): -0.000319441 device_current
I(V1): -0.0319441 device_current
As the voltage V2 is 5V and V3 is 5V, thus the transistor with V3 works or it
act as short as it is greater than the 0.7V and the transistor with V2 also works
as the npn transistor requires only 0.7V but it is 5V so both are short so the
overall output is obtained as both work together for the output across the led and
the led glows.
3) OR Gate:
a) For V2 is 0V and V3 is 0V
Output:

9. --- Operating Point ---
V(n001): 9 voltage
V(n002): 9.0001e-008 voltage
V(n004): 9.40849e-013 voltage
V(n007): 9.0001e-008 voltage
V(n003): 0 voltage
V(n006): 0 voltage
Ic(Q2): 9.0002e-012 device_current
Ib(Q2): -9.0001e-012 device_current
Ie(Q2): -1.0009e-016 device_current
Ic(Q1): 9.0002e-012 device_current
Ib(Q1): -9.0001e-012 device_current
Ie(Q1): -1.0009e-016 device_current
I(D1): 1.3046e-024 device_current
I(R3): 2.00181e-016 device_current
I(R2): -9.0001e-012 device_current
I(R1): -9.0001e-012 device_current
I(V3): 9.0001e-012 device_current
I(V2): 9.0001e-012 device_current
I(V1): -1.80004e-011 device_current
As the V2 and V3 is 0V so the both transistor does not works as they requires
only 0.7V in the npn transistors so both act as the open and the 9V source is not
passed on to these and at last the current can’t reaches the led diode and does
not glow and output is not obtained.
b) For V2 is 5V and V3 is 0V

10. Output:
--- Operating Point ---
V(n001): 9 voltage
V(n002): 1.61173 voltage
V(n004): 0.746377 voltage
V(n007): 9.74648e-008 voltage
V(n003): 5 voltage
V(n006): 0 voltage
Ic(Q2): 9.0001e-012 device_current
Ib(Q2): -9.74648e-012 device_current
Ie(Q2): 7.46378e-013 device_current
Ic(Q1): 0.0338827 device_current
Ib(Q1): 0.000338827 device_current
Ie(Q1): -0.0342216 device_current
I(D1): 0.0340641 device_current
I(R3): 0.000158804 device_current
I(R2): -9.74648e-012 device_current
I(R1): 0.000338827 device_current
I(V3): 9.74648e-012 device_current
I(V2): -0.000338827 device_current
I(V1): -0.0338827 device_current
As the V3 is 0V and V2 is 5V so the transistor with V3 does not works as it
requires only 0.7V in the npn transistors so it act as the open and the transistor
with V2 works as it is greater than 0.7V and got short so the 9V source is passed

11. on to the one of transistor and at last the current reaches the led diode and it
glow and output is obtained.
c) for V2 is 0V and V3 is 5V
Output:
--- Operating Point ---
V(n001): 9 voltage
V(n002): 9.74648e-008 voltage
V(n004): 0.746377 voltage
V(n007): 1.61173 voltage
V(n003): 0 voltage
V(n006): 5 voltage
Ic(Q2): 0.0338827 device_current
Ib(Q2): 0.000338827 device_current
Ie(Q2): -0.0342216 device_current
Ic(Q1): 9.0001e-012 device_current
Ib(Q1): -9.74648e-012 device_current
Ie(Q1): 7.46378e-013 device_current

12. I(D1): 0.0340641 device_current
I(R3): 0.000158804 device_current
I(R2): 0.000338827 device_current
I(R1): -9.74648e-012 device_current
I(V3): -0.000338827 device_current
I(V2): 9.74648e-012 device_current
I(V1): -0.0338827 device_current
As the V2 is 0V and V3 is 5V so the transistor with V2 does not works as it
requires only 0.7V in the npn transistors so it act as the open and the transistor
with V3 works as it is greater than 0.7V and got short so the 9V source is
passed on to the one of transistor and at last the current reaches the led diode
and it glow and output is obtained.
d) For V2 and V3 is 5V
Output:
--- Operating Point ---
V(n001): 9 voltage
V(n002): 1.62944 voltage
V(n004): 0.764227 voltage
V(n007): 1.62944 voltage
V(n003): 5 voltage
V(n006): 5 voltage

13. Ic(Q2): 0.0337056 device_current
Ib(Q2): 0.000337056 device_current
Ie(Q2): -0.0340426 device_current
Ic(Q1): 0.0337056 device_current
Ib(Q1): 0.000337056 device_current
Ie(Q1): -0.0340426 device_current
I(D1): 0.0679227 device_current
I(R3): 0.000162601 device_current
I(R2): 0.000337056 device_current
I(R1): 0.000337056 device_current
I(V3): -0.000337056 device_current
I(V2): -0.000337056 device_current
I(V1): -0.0674112 device_current
As the both the input is 5V so the both transistor works as they requires only
0.7V in the npn transistors so both act as the short and the 9V source is passed
on to these and at last the current reaches the led diode and makes it glow and
output is obtained.
4) NOR Gate
a) For V2 and V3 is 0V

14. Output
--- Operating Point ---
V(n001): 0.698262 voltage
V(n003): 6.98362e-009 voltage
V(n007): 6.98362e-009 voltage
V(n004): 0 voltage
V(n006): 0 voltage
V(n002): 6 voltage
Ic(Q2): 6.98462e-013 device_current
Ib(Q2): -6.98362e-013 device_current
Ie(Q2): -1.00007e-016 device_current
Ic(Q1): 6.98462e-013 device_current
Ib(Q1): -6.98362e-013 device_current
Ie(Q1): -1.00007e-016 device_current
I(D1): 0.00530174 device_current
I(R3): -0.00530174 device_current
I(R2): -6.98362e-013 device_current
I(R1): -6.98362e-013 device_current
I(V1): -0.00530174 device_current
I(V3): 6.98362e-013 device_current
I(V2): 6.98362e-013 device_current

15. As the V2 and V3 is 0V so the both transistor does not works as they requires
only 0.7V in the npn transistors so both act as the open and the 9V source is not
passed on to these and passed from the led and it glows and output is obtained.
b) For V2 is 0V and V3 is 5V
Output:
--- Operating Point ---
V(n001): 0.0759632 voltage
V(n003): 7.60579e-010 voltage
V(n007): 0.823145 voltage
V(n004): 0 voltage
V(n006): 5 voltage
V(n002): 6 voltage
Ic(Q2): 0.00592404 device_current
Ib(Q2): 0.000417685 device_current
Ie(Q2): -0.00634172 device_current
Ic(Q1): 7.61526e-014 device_current
Ib(Q1): -7.60579e-014 device_current
Ie(Q1): -9.46979e-017 device_current

16. I(D1): 2.54539e-013 device_current
I(R3): -0.00592404 device_current
I(R2): 0.000417685 device_current
I(R1): -7.60579e-014 device_current
I(V1): -0.00592404 device_current
I(V3): -0.000417685 device_current
I(V2): 7.60579e-014 device_current
As the V2 is 0V and V3 is 5V so the transistor with V2 does not works as it
requires only 0.7V in the npn transistors so it act as the open and the transistor
with V3 works as it is greater than 0.7V and got short so the current can’t pass
through the transistors across V2 but pass through the transistor across V3, so
the current pass to this short transistor and can’t pass to the Led thus the led
does not glows and output is zero.
c) V2 is 5v and V3 is 0V
Output
--- Operating Point ---
V(n001): 0.0759632 voltage
V(n003): 0.823145 voltage
V(n007): 7.60579e-010 voltage
V(n004): 5 voltage
V(n006): 0 voltage

17. V(n002): 6 voltage
Ic(Q2): 7.61526e-014 device_current
Ib(Q2): -7.60579e-014 device_current
Ie(Q2): -9.46979e-017 device_current
Ic(Q1): 0.00592404 device_current
Ib(Q1): 0.000417685 device_current
Ie(Q1): -0.00634172 device_current
I(D1): 2.54539e-013 device_current
I(R3): -0.00592404 device_current
I(R2): -7.60579e-014 device_current
I(R1): 0.000417685 device_current
I(V1): -0.00592404 device_current
I(V3): 7.60579e-014 device_current
I(V2): -0.000417685 device_current
As the V3 is 0V and V2 is 5V so the transistor with V3 does not works as it
requires only 0.7V in the npn transistors so it act as the open and the transistor
with V2 works as it is greater than 0.7V and got short so the current can’t pass
through the transistors across V3 but pass through the transistor across V2 , so
the current pass to this short transistor and can’t pass to the Led thus the led
does not glows and output is zero
d) For V2 and V3 is 5V

18. Output
--- Operating Point ---
V(n001): 0.0589801 voltage
V(n003): 0.808309 voltage
V(n007): 0.808309 voltage
V(n004): 5 voltage
V(n006): 5 voltage
V(n002): 6 voltage
Ic(Q2): 0.00297051 device_current
Ib(Q2): 0.000419169 device_current
Ie(Q2): -0.00338968 device_current
Ic(Q1): 0.00297051 device_current
Ib(Q1): 0.000419169 device_current
Ie(Q1): -0.00338968 device_current
I(D1): 1.46777e-013 device_current
I(R3): -0.00594102 device_current
I(R2): 0.000419169 device_current
I(R1): 0.000419169 device_current
I(V1): -0.00594102 device_current
I(V3): -0.000419169 device_current
I(V2): -0.000419169 device_current
As the V2 is 5V and V3 is 5V so the transistor with V2 does works as it requires
only 0.7V in the npn transistors so it act as the short and the transistor with V3

19. also works as it is greater than 0.7V and got short so the current pass through
the transistors across V2 and V3 both and can’t pass to the Led thus the led does
not glows and output is zero.
5) NAND Gate
a) For V2 and V3 is 0V
Output
--- Operating Point ---
V(n001): 0.698262 voltage
V(n003): 6.98412e-009 voltage
V(n005): 4.97145e-005 voltage
V(n007): 4.99065e-013 voltage
V(n004): 0 voltage
V(n006): 0 voltage
V(n002): 6 voltage
Ic(Q2): 5.00986e-017 device_current
Ib(Q2): -4.99065e-017 device_current

20. Ie(Q2): -1.92024e-019 device_current
Ic(Q1): 6.98462e-013 device_current
Ib(Q1): -6.98412e-013 device_current
Ie(Q1): -5.00986e-017 device_current
I(D1): 0.00530174 device_current
I(R3): -0.00530174 device_current
I(R2): -4.99065e-017 device_current
I(R1): -6.98412e-013 device_current
I(V1): -0.00530174 device_current
I(V3): 4.99065e-017 device_current
I(V2): 6.98412e-013 device_current
As the voltage V2 and V3 is 0V, thus the both transistor don’t work as the npn
transistor requires 0.7V, so becomes open and the whole current pass through
the led and it glows and the output is obtained
b) For V2 is 5V and V3 is 0V
Output:
--- Operating Point ---
V(n001): 0.699932 voltage
V(n003): 1.44714 voltage
V(n005): 0.70019 voltage

21. V(n007): 7.00289e-009 voltage
V(n004): 5 voltage
V(n006): 0 voltage
V(n002): 6 voltage
Ic(Q2): 7.0039e-013 device_current
Ib(Q2): -7.00289e-013 device_current
Ie(Q2): -1.00007e-016 device_current
Ic(Q1): -0.000355281 device_current
Ib(Q1): 0.000355286 device_current
Ie(Q1): -4.72982e-009 device_current
I(D1): 0.00565535 device_current
I(R3): -0.00530007 device_current
I(R2): -7.00289e-013 device_current
I(R1): 0.000355286 device_current
I(V1): -0.00530007 device_current
I(V3): 7.00289e-013 device_current
I(V2): -0.000355286 device_current
As the voltage V2 is 5V and V3 is 0V, thus the transistor with V2 works or it
act as short as it is greater than the 0.7V but the transistor with V3 don’t works
as the npn transistor requires 0.7V but it is zero so it is open but overall this
does not work as it is open so the whole current is passed to the led and it glows
and output is obtained.
c) For V2 is 0V and V3 is 5V

22. Output:
--- Operating Point ---
V(n001): 0.698262 voltage
V(n003): 7.16291e-009 voltage
V(n005): 0.0179282 voltage
V(n007): 0.769397 voltage
V(n004): 0 voltage
V(n006): 5 voltage
V(n002): 6 voltage
Ic(Q2): -1.78777e-014 device_current
Ib(Q2): 0.00042306 device_current
Ie(Q2): -0.00042306 device_current
Ic(Q1): 6.98412e-013 device_current
Ib(Q1): -7.16291e-013 device_current
Ie(Q1): 1.78787e-014 device_current
I(D1): 0.00530174 device_current
I(R3): -0.00530174 device_current
I(R2): 0.00042306 device_current
I(R1): -7.16291e-013 device_current
I(V1): -0.00530174 device_current
I(V3): -0.00042306 device_current
I(V2): 7.16291e-013 device_current

23. As the voltage V2 is 0V and V3 is 5V, thus the transistor with V3 works or it
act as short as it is greater than the 0.7V but the transistor with V2 don’t works
as the npn transistor requires 0.7V but this circuit does not work as it is open
and the whole current pass to the led and it glows and output is obtained.
d) For V2 and V3 is 5V
Output
--- Operating Point ---
V(n001): 0.153564 voltage
V(n003): 0.900241 voltage
V(n005): 0.0774538 voltage
V(n007): 0.824386 voltage
V(n004): 5 voltage
V(n006): 5 voltage
V(n002): 6 voltage
Ic(Q2): 0.00625641 device_current
Ib(Q2): 0.000417561 device_current
Ie(Q2): -0.00667397 device_current
Ic(Q1): 0.00584644 device_current
Ib(Q1): 0.000409976 device_current
Ie(Q1): -0.00625641 device_current

24. I(D1): 3.93205e-012 device_current
I(R3): -0.00584644 device_current
I(R2): 0.000417561 device_current
I(R1): 0.000409976 device_current
I(V1): -0.00584644 device_current
I(V3): -0.000417561 device_current
I(V2): -0.000409976 device_current
As the voltage V2 and V3 is 5V, thus the both transistor works as the npn
transistor requires 0.7V, and got shorted, thus the current pass through it and not
to the led so no output is obtained as led does not glows.
By
Lijin G Varghese