There are many logical families such as RTL, DTL, ECL, CMOS etc.
All have their advantages and disadvantages. However, in the past decade, CMOS is ruling the electronics field because of its SIZE and POWER.
CMOS(Complementary metal oxide semiconductor) has ZERO static power dissipation. However, The switching speed of CMOS is very low compared to BJT, and also CMOS cannot drive large current to the load. In order to drive large current to the load, the size of MOS needs to be significantly increased.
In order to incorporate the advantages of both CMOS and BJT, new logical family came into existence and that logical family is called BICMOS. In this post we will see the working of BICMOS NAND GATE and appreciate the work.
This is the two input BICMOS NAND gate.
It consists of
1) 2 PMOS PA and PB
2) 4 NMOS, NA1, NB1, NA3, NB3
3) Two BJT’s QP and Q0
A B Y
0 0 1
0 1 1
1 0 1
1 1 0
Purpose of Various devices
- PA, PB, NA1 and NB1 are used for logical purposes(Just like in CMOS)
- N2, NB3 and NA3 are used to remove base charge from the transistors.
Why to remove the base charge anyway?
- For high switching speeds of the BJT’s, we need to remove the base charge from the transistor. In order to remove the charge from the base of the transistor we need a mechanism, that can be achieved by using these 3 NMOS.
- If we do not remove the base charge, the transistor will be in ON state and takes a lot of time to go to the OFF state. If we do not remove the base charge, the half purpose of BICMOS will be lost.
Case 1 : Both A and B are low.
If both the inputs VA and VB(Refer the circuit) are low(0). PA and PB will be ON and the base of QP will be high. Thus the top BJT(Q) will be ON which pulls the output UP . If there is a capacitive load. The output current will be almost 101 times the base current i.e it will be 101*IB(Assuming beta of transistor is 100). (if there was no BJT, the output current would be just IB).
Also, note that N2 is getting input from the base of QA transistor. Since the base of QP is HIGH(say 5v), it will turn on N2 . Since N2 is ON, it will PULL THE BASE CHARGE OUT OF Q0 transistor. So, if both the inputs are low, QP is ON and Q0 is OFF. It is very important to appreciate the role of N2 here which facilitates high switching speed.
Case 2 :Both A and B are high
NB3 and NA3 NMOS make sure that QP is in OFF state. PA and PB are OFF. NB1, NA1 will be ON. The output is discharged via NB1, NA1 and Q0(High Speed). N2 will be effectively out of circuit in this case. So, Q0 is ON and QP is OFF.
I have discussed the 2 cases. You can correlate with the other 2 inputs.
Disadvantages of BICMOS
1) Fabrication cost is high
2) Due to VBE(Base to emitter voltage, 0.7v approximately) of BJT’s , desirable performance is not obtained when the BICMOS gates are operated at lesser voltages (3V, 2.4V) etc.