Hi all
I have always a question in mind.

WITHOUT considering the price, under what condition a transistor will be chosen, instead of an opamp?
A good opamp has far lower noise and distortion.

Richie

 

When a switch is needed instead of an amplifier

Save board space when an amplifier is needed

Speed of transitions (slew rate in op-amps)

When dealing with medium to large currents (> 100mA).

There's more reasons, but those come off the top of my head as glaring examples.

 

Many opamps are limited to 20mA to 30mA source or sink output current. A typical small-signal transistor may be capable of 50mA up to 500mA source/sink current, depending on the type and how it's connected.

Transistors are generally far cheaper than opamps, and if only one is required, usually take less board space. Cost is always a consideration.

 

Short answer: When you can't quite get what you need in a monolithic device, but is still achievable with discreet parts.

In a two applications, I was not able to find an opamp with the required offset stability, low enough open loop gain to prevent oscillation, and high voltage capability to allow a high enough compliance voltage in a linear magnetic deflection amplifier, and so ended up making a precision bipolar amplifier out of discreet parts.

Applications such as these are becoming scarce now that there is such a huge collection of widely varied available opamps.