LVPECL output circuit explanation

Question

I don't understand the circuit itself, how we will get VOH and VOL on the outputs. There is 2 modes one mode is when Q1 is conducting and the other mode is when Q2 is conducting , how from here how will we get the VOH and VOL in the 2 outputs?

Answer 1

A basic simulation of the circuit can be set up right in the CircuitLab circuit editor available in both the question and answer editors.

^{simulate this circuit – Schematic created using CircuitLab}

Q1 and Q2 base voltages, and Q3 and Q4 emitter voltages are plotted below.

Q1 and Q2 form a differential-input pair. That's what makes it work, pretty much.

The inputs have no differential voltage across them, Q1 and Q2 conduct equal currents, the voltage drop across R1 and R2 is similar, and is then buffered by the emitter followers Q3 and Q4, and there's not much voltage difference between Q3.E and Q4.E.

As soon as the differential input voltage is non-zero, Q1 and Q2 conduct different currents, since their B-E voltages differ. Voltage difference between the bottom of R1 and R2 grows, and is buffered by Q3 and Q4, so the difference across R3 and R4 is higher too.

There is 2 modes one mode is when Q1 is conducting and the other mode is when Q2 is conducting

That is a very "textbook" approximation and doesn't quite reflect what's happening. Q1 and Q2 always conduct. The ratio of their currents, when fully settled, is usually 1:5-1:20, depending on the details of implementation.

In the simulation above, the collector currents of Q1 and Q2 look as follows:

This ratio doesn't change if we replace the tail current source with a tail resistor of about 100Ω - it's still about 1:10.