Ciruit description
The circiuit is to 95% based on the SSB01. 1I 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.
LED bias network
The old design SSB01 used a plain resistor to set the bias through the Q1, Q3, Q5 and Q2, Q4, Q6 but the disadvantage is that the circuit is more sensitive against varying supply voltage. The LED circuit (see page 2 of the schematic) is a low noise current source formed by two halfs consisting of cascoded current source where the LED serves as a voltage reference. Red colour is recommended but some resistors need to be recalculated if the same quiesent currents are wanted.
The current in the first stages is set by Q1, Q3 and Q2, Q4 along with R6 and R7. Transistors are used 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. 4-6 mA.
Class A in the buffer
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. I have experimented with different settings but it becomes more difficult an unpredictable with higher settings. I support only the setting I have. The rest is left up to you.
Input filter
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). -3 dB limit, see technical data below.
No current limitation
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.
