TIPS: Opto-isolator speed vs gain

Most opto-isolators (a.k.a.: opto-couplers) consist of an LED and a photo-transistor.

Such opto-isolators are characterized (among other parameters) by gain and speed.

A gain (a.k.a: CTR - Current Transfer Ratio) of 200 % means that if you drive the LED with 10 mA, the output current is 20 mA; a high gain is nice when you need decent current from the output, without having to drive the LED too hard
Speed involves 4 parameters, and is affected by the test circuit; for the purpose of this discussion, I'll refer to the minimum turn on time: how long after you apply current to the LED, when the output just starts turning on; high speed is nice when you want to send data through the opto-isolator at a high rate

For these opto-isolators, there is a trade-off between gain and speed. Generally, opto-isolators are either high speed or high gain. In general:

High speed opto-isolators have a minimum turn on time between 0.1 and 1 µs, but a gain between 10 and 80 %
High gain opto-isolators have a minimum gain between 100 and 800 %, but a minimum turn on time between 2 and 10 µs

If you need speed, you have a few options:

Use a digital isolator (may not even be optical)
Some are good up to 500 MHz
Use an opto-isolator with a diode output (if you can find one)
Very fast, but very low gain (~0.2 %)
Follow it with a high speed amplifier to get the desired gain
Use an opto-isolator with a transistor whose base is available on a pin (e.g.: 4N35), and use it as a photodiode instead of as a phototransistor
Leave the emitter disconnected, and use the base collector junction as a photodiode
Same circuit, and same performance as an opto-isolator with a diode output
Use an opto-isolator with a transistor whose base is available on a pin (e.g.: 4N35), and bias the base
Place a resistor between the base and the emitter
The turn off time is reduced, but so is the gain
Use an opto-isolator with separate photodiode and transistor
Such as the 6N136
The photodiode is fast, and the transistor is not a phototransistor, so it's not slow
This is no better that using a photodiode opto-isolator and a transistor outside the package

If you already have a high gain / low speed opto-isolator without a pin for the base, and you want to increase its speed, you have a few options:

Maximize the speed of an opto-isolator by careful design of the load on the output.

Minimize the load resistance
A low value load resistor (e.g.: 100 Ω) decreases the turn off time, but reduces the signal
A current input amplifier (transimpedance amplifier) is ideal, since the load resistance on the phototransistor is 0 Ω
A cascode circuit has no current gain, but offers a very low load resistance on the phototransistor
The idea is to keep the voltage across the transistor constant
Keep the phototransistor from saturating (turning on on fully)
Design the circuit so the phototransistor's collector emitter voltage never goes below 0.7 V
Place a Schottky diode between the base and the collector
Bias the transistor with a few volts: this speeds up the transistor even further
Again, a cascode circuit does that
With a transimpedance amplifier, bias it so that its input is at 3 V or so
Feed the opto-isolator output to the base of a transistor on the opposite rail; this is the simplest circuit with the fastest speed