Ciruit description

I have chosen to use current generators with cascodes as emitter loads. How much good the cascodes do is unknown at the moment. The only obvious disadvantage is a little bit reduced output voltage swing, otherwise only positive things.

The current in the first stages is set by Q1, Q3 and Q2, Q4 along with R6 and R7. I have transistors as diodes just to make it simpler when it comes to ordering parts. The current is 0.65/220= 3 mA and the output stage will have approx. 6 mA. The class A circuit will have 1.5 mA. My choice of currents is "average", because you both increase them och also decrease them. The max current is set by the max allowable power dissipation of the transistors. The small SOT23 types can take 310mW or so and the SOT223, output transistors can dissipate 1.3 W but not in this application I imagine. More current mean slightly higher speed.

The input filter R5, C1 may be necessary if you are having trouble with RF interference. The values can be set to almost anything. The frequency is calculated by f= 1/(2*pi*R5*C1).

Please note that this buffer has no current limitation in order to protect the circuit so a short circuit may damage the output transistors. The buffer is intended to be used in controlled environments. If your load is unknown you may add a resistor at the output, 220 ohms or higher.