# Output of a NAND gate

Above is a TTL totem pole output NAND gate. There is a 120 ohm pull up resistor there. Since it is called pull up, can we say that the HI output will be connected to a very high input impedance? If so what can it be as an example? Aren't NAND gates in an IC are connected to each other by being ones output is other's input? I mean in the figure the output will be the input of another gate right? If so HIGH will not be 5V since there will not be input impedance unless it is not connected to a very high resistance.

Where is this NAND gate's input coming from and where is the output going to? If the output is 5V isnt it high for a new TTL gate input and if is not 5V why do we call the resistor pull up in there?

• the OUTPUT is low impedance, the INPUT is high impedance. If one drives another, then it results in nice low current/power. – KyranF Oct 24 '14 at 22:04
• Also the 120 Ohm resistor is not a pull up, it's a series current limiting resistor for probably a very low size/power rating transistor for the "push" stage of the totem pole driver. – KyranF Oct 24 '14 at 22:05
• Imagine what would happen if there was no resistor there? The output stage of the totem pole would be almost a short circuit (other than the limiting factor of the DC Current gain of the BJT.. but whatever) and things would blow up if the output was shorted to ground for example. In this example, the NAND gate can "source" ~41mA. – KyranF Oct 24 '14 at 22:08
• lets call this transistor above T1 and say we feed the output in this figure to another transistor T2's input. what will be the high impedance you are mentioning? the T2's emitter inputs without any extra resistor? and what will be the low output impedance of T1? 120 ohm of the T1? could u expound on that a bit? – user16307 Oct 24 '14 at 22:10
• The input to this circuit is high impedance because Q1 is acting in high resistance, to let through the InA and InB signals. If you chain these devices input to output etc, the 120 ohm value will not even matter – KyranF Oct 25 '14 at 10:17

The primary purpose of the 120$\Omega$ resistor is to reduce the current spikes when the output switches (when Qo and Qp are both on simultaneously for a brief moment). See, for example, here. It's a component part of the (active) pullup circuit, but it's not a 'pullup resistor'.
Totem pole outputs like this one use an active pullup, which is Qp, Rc and Rcp. When Qs is 'off', the base of Qp is pulled to Vcc by Rc, so the effective pullup resistance is limited by the collector resistance Rcp - so it's about 120$\Omega$, meaning that for a 50pF load, the time constant is about 6ns. Without the collector resistor it would behave more like a few ohms (1.6K divided by the current gain of Qp).