2-input TTL NAND gate

A TTL NAND gate (7400) is built with bipolar transistors: a multi-emitter input transistor, a phase splitter and a totem-pole output.

              +5 V
   +-----------+----------+
   |           |          |
 R1 4k      R2 1.6k    R4 130
   |           |          |
   b           +-----+    c
A--e  Q1       |     +--b Q3
B--e     c--+  c          e
            +--b Q2       |
               e         D1
               |          |
               +-----+    +---- Y
               |     |    c
             R3 1k   +--b Q4
               |          e
   GND --------+----------+

(b, c, e = base, collector, emitter; Q1 is a multi-emitter transistor.)

Circuit parts (standard 7400):

• Q1 (multi-emitter input transistor): one emitter per input; works like an AND of diodes.
• Q2 (phase splitter): gives opposite signals at its collector and emitter to drive the output pair.
• Q3, D1, Q4 (totem-pole output): Q3 is the active pull-up, Q4 the pull-down; D1 makes sure Q3 stays OFF when Q4 is ON. R4 = 130 Ω limits the current spike while switching.

Operation:

• Any input LOW (≈ 0.2 V): that base–emitter junction of Q1 conducts, so Q1's base sits at about 0.2 + 0.7 = 0.9 V. This is too low to forward bias Q1's base–collector junction together with the base–emitter junctions of Q2 and Q4 (that needs about 3 × 0.7 = 2.1 V), so Q2 and Q4 are OFF. Q2's collector is pulled up through R2, so Q3 conducts as an emitter follower and the output is HIGH (typically about 3.4 V, at least 2.4 V guaranteed).
• All inputs HIGH: all emitter junctions of Q1 are reverse biased. Current from R1 flows through Q1's base–collector junction into the base of Q2 (Q1's base rises to about 2.1 V), so Q2 and Q4 saturate. The output is pulled LOW (about 0.2 V). Q2's collector is only about 0.9 V, which cannot forward bias both Q3's base–emitter junction and D1, so Q3 is OFF.

Hence $Y = \overline{AB}$.

Key values: $V_{OH} \approx 3.4$ V (≥ 2.4 V), $V_{OL} \approx 0.2$ V (≤ 0.4 V), propagation delay ≈ 10 ns, power ≈ 10 mW per gate, fan-out 10.