Perhaps, the author is just trying clarify a point without making an exact calculation.ġ) I have understood "push-pull" to imply that there are active devices driving the circuit high and low, This can be two devices of the same polarity driving though a centre-tapped transformer. How are these voltages, 2.1V, 1.4V, and 0.7V calculated? They add up to 4.2V and not to 5V. It says Vcc=5V but wouldn't it then require HIGH to be greater than 5V if the base emitter junction of Q1 transistor is to be reverse biased. when the voltage goes below the ground level temporarily, the current flows from ground terminal through D1 to the input? This way current wouldn't flow through Q1 transistor.
You can see here four circuits and all of them look different from each other considering the layout of components but function-wise they are pretty much the same in the context of given circuit.Ĭircuit #1 and #2 were taken from this webpage.ĭoes this mean that when there is a negative spike at the input, i.e. The totem-pole circuit also does the same thing, in my opinion, making the output low or high but its use is mostly restricted to turning on/off of FET transistors. Such a combination is used to pull the output voltage toward ground or toward the supply voltage in other words low and high.
It is my understanding that when the word pull-up/pull-down is used, it refers to combination of a resistor and switch such as this one. I believe that the words pull-up/pull-up network and push-pull network are synonymous in the context of circuits. That drive capability guarantees each TTL output the capability to drive 10 TTL inputs with a guaranteed noise margin of 400mV or greater.Could you please help me with the queries below? Thank you.
TTL outputs will be 400mV or less when low (and sinking 16mA or less), and 2.4V or more when high (and sourcing 400uA or less). TTL inputs are defined as 'low' if they are less than or equal to 800mV and 'high' if they are greater than or equal to 2V. Without the collector resistor it would behave more like a few ohms (1.6K divided by the current gain of Qp). 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. Totem pole outputs like this one use an active pullup, which is Qp, Rc and Rcp. It's a component part of the (active) pullup circuit, but it's not a 'pullup resistor'. 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).
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