What is this transistor configuration? (differential input)

[Manx]

The following transistor configuration is used at inputs of a DYI EEG device http://openeeg.sourceforge.net/doc/modeeg/modEEGamp-v1.0.png

What does it do?

[Paul Rose]

There are some web links in the schematic
see  http://openeeg.sourceforge.net/doc/modeeg/modeeg_design.html

Look under section 2.3 "Detailed description"

Protection circuit

The first stop after entering the analog board via the EEG connectors, is the protection circuit.

At the front sits three capacitors, C210, C206 and C207 that are supposed to suppress radio-frequency signals entering the system via the electrode cable. They are followed by a netowork of transistors (Q202, Q204, Q206 Q208) and some resistors (R203, R204 and R209 to R212).

The transistors are not actually used as such, instead they act as clamping diodes.

When the voltage over one of these "diodes" exceeds about 0.2 volts, they start to conduct current. Below that level, they are basically open circuits, only conducting a few picoamperes. If the voltage reaches about 0.7 volts, they begin to conduct in earnest. With the help of the resistors they prevent the voltage from ever going above 0.7 volts. They also limit the current going through a user, in case of short circuit between a power rail and an amplifier input pin.

[David Hess]

My, but that is clever.

If R205 and R208 were shorted, then you would have anti-parallel connected pairs of transistor base-emitter junctions providing shunt protection to the virtual ground on the output which is a very good way to make a fast and low leakage shunt to limit the signal voltage.

With R205 and R208 included, the base-emitter current is limited, protecting the base-emitter junctions which are relatively delicate, and the transistor gain redirects the current through the collectors.  I wonder if they had a problem with large ESD spikes damaging the base-emitter junctions.  They could have just used the base-collector junctions which are much tougher albeit slower, but not slow enough to matter.

C204 and C205 keep RF out of the base-emitter junctions which would otherwise be rectified causing DC voltage shifts.

[AndersJ]

How is this better than two antiparallell low leakage diodes?

[SeanB]

Lower leakage, and a much lower cost, plus in the most case using a TO92 package transistor, is they are insensitive to light increasing leakage current. Plus as well higher current handling capacity, unlike the low leakage diodes. As well if you run then so the emitter is the positive lead you get a poorly defined zener voltage of around 6V, but current handling equal to the transistor if there is need for it, and leakage current that i typically only available with very expensive devices. If you need lower leakage you go for junction FET's with the emitters joined together, and use the gate as low capacitance diode in the clamp, though there you will need a power supply clamp to handle the charge injected into it during clamping.

[David Hess]

How is this better than two antiparallell low leakage diodes?

Cost is lower, availability is better, and transistor current gain multiplies current handling capability.

I assume they evaluated a simpler design, maybe using only the transistor base-emitter junctions as low leakage diodes, and found performance lacking.