# How to calculate the collector and emitter resistors in a differential amplifier with transistors

I'm refreshing my electronics knowledge a bit (after 30+ years), and I'm trying to build an audio amplifier from scratch without integrated circuits (transistors, and mosfets only).

I'm trying to build the input stage, which is a differential amplifier using 2 transistors. I'm currently simulating it with LTSpice, but I seem to amplify only the positive part of my input signal. I guess it's because of the values of the emitter and collector resistors.

I understand the principle of a diff amplifier, but I'm struggling with the practical setup.

Can somebody explain to me (or point me to a site) how to calculate those resistors?

I guess there should be current flowing through R2 while out1 should be at ground level, if no signal is applied?

I included my schematic and the waveforms.

Thanks a lot!

• There are plenty of books and courses that explain how to do circuit design. An BJT diffpair with no degeneration can only handle an input voltage of around 50 mV so you're over steering this pair by a factor 10. Commented Jan 21, 2021 at 16:47
• The working principle of a "good" diff. amplifier requires a large resistor in the common emitter leg - much larger than the collector resistors. Remember: Very often we treat this resistor - together with the supply voltage - as a current source.
– LvW
Commented Jan 21, 2021 at 17:19
• @Bimpelrekkie I was expecting oversteering, and I hoped to see a square wave between +20 and -20V. Now I only see the upper half and a almost nothing in the neg part. Next would be adding feedback. Commented Jan 21, 2021 at 19:14
• When do you expect the voltage to be -20 V? The lowest value at out1 occurs when in1 is at a the top of the sinewave (+0.5 V). Now investigate that! Copy this schematic to a new sheet, then remove V3 and replace it with a DC voltage source of 0.5 V. Then do a DC operating point simulation. With the result you can observe the state of the circuit as if it is "frozen in time" at the top of the sinewave. ... Commented Jan 21, 2021 at 19:41
• ...What are the values of the voltages of Q1 like $V_{BE}$ and $V_{CE}$. What does that mean for the operation of the transistor? Suppose there was -20 V at out1, would that be OK for the transistor or would something blow up? Think of the diode model of the bipolar transistor. Commented Jan 21, 2021 at 19:41