Circuit measures microvolts in the presence of volts

Because one ought not to rely on a warm transistor to pass no more than a few nanoamperes, Q4 and Q5 with R20 form a series-shunt power switch. Because Q4's off-current is very small, it only drops a tiny voltage across R20. Q5 then passes very little current, due to the fact that it has little voltage across it. D8 is chosen to have low leakage to avoid unwanted leakage current in the power-switch output. Baker clamps are applied to both Q4 and Q5 to ensure prompt switching.