Hi,

Here is a simple trick to improve the performance of the two-transistor CCS.

This type of current source has a good regulation vs. variations of the output voltage (= high output impedance), but behaves rather poorly wrt. variations of the auxiliary (bias) voltage (= non-zero transconductance).

By adding a single compensation resistor, a huge gain can be made.

In this example, a 10mA current sink, resistor R4 has been added.

The circuit on the right is the regular CCS used as the reference for comparison.

Without R4, the current variation amounts to 24µA for 1V.

With R4, this variation is only 0.27µA, almost a 100 to 1 improvement.

The value has been fine-tuned for the optimum, and R2 also had to be altered slightly to keep the same current.

I have found the values empirically, it should be possible to derive a formula.

I think it should look something like this:

The ratio of R4 to R2 equals 2(Iout/Ibias)(Vbias/Vbe) (to be checked, at first sight, it seems to work).

It is not necessary to use exact values: the second example uses the nearest standard values, and the delta current has increased to 0.7µA, but it is still a worthwhile gain.

With this method, the PSRR of an amplifier could be improved by 20 to 40dB.

Here is a simple trick to improve the performance of the two-transistor CCS.

This type of current source has a good regulation vs. variations of the output voltage (= high output impedance), but behaves rather poorly wrt. variations of the auxiliary (bias) voltage (= non-zero transconductance).

By adding a single compensation resistor, a huge gain can be made.

In this example, a 10mA current sink, resistor R4 has been added.

The circuit on the right is the regular CCS used as the reference for comparison.

Without R4, the current variation amounts to 24µA for 1V.

With R4, this variation is only 0.27µA, almost a 100 to 1 improvement.

The value has been fine-tuned for the optimum, and R2 also had to be altered slightly to keep the same current.

I have found the values empirically, it should be possible to derive a formula.

I think it should look something like this:

The ratio of R4 to R2 equals 2(Iout/Ibias)(Vbias/Vbe) (to be checked, at first sight, it seems to work).

It is not necessary to use exact values: the second example uses the nearest standard values, and the delta current has increased to 0.7µA, but it is still a worthwhile gain.

With this method, the PSRR of an amplifier could be improved by 20 to 40dB.