Constant current sources are basic building blocks of almost every analog audio amplifier. There are countless circuits and each has its own unique properties. In this article I will compare a small selection of different variants with each other in order to identify their advantages. I will use the best ones for my next audio amplifier. Performance criteria considered in this comparison are: Thermal drift, supply voltage dependence, PSRR, step response and output impedance.
In general, both source and sink were investigated. For simplicity, only the schematic of the source is shown.
LED reference CCS
Zener reference CCS
Amplified negative feedback CCS
Complimentary CCS with cascodes
All current sources were set to 10mA at 50°C for comparison. This temperature drift simulation is merely a crude approximation because in reality, the components are unlikely to share same temperature. However, it gives a hint how the circuits may behave.
The simulation shows that all current sources that use a LED as reference, show the lowest temperature drift. The Zener does show more drift and the ANF CCS shows most drift. All CCS except the ANF CCS show a positive temperature coefficient.
For this investigation, all CCS were set to deliver 10mA at 100V supply voltage. The CCS using the LED or Zener and also the ANF type output current is strongly dependent on the voltage across the CCS. Here, the ANF performs best among those three. The complementary constant current sources show almost no dependency of the output voltage on supply voltage dependency. Among those two, the cascoded CCS performs slightly better. With only 3V of voltage headroom required, the complimentary CCS is a stellar performer and requires only twice the number of super low cost components.
PSRR is related to the supply voltage dependency of the output current observed earlier. While the simple current sources using either a LED or Zener as reference or the ANF type CCS show better high frequency PSRR above a few hundret kHz, the complimentary CCS show significant improvement at lower frequencies, which is more relevant to audio applications.
From experience, I can tell you that you get the most PSRR for the buck with very careful attention to the quality of constant current sources that you might employ, particularly in creating voltage references for them.
Nelson Pass on DIY Audio
The simple CCS suffer from the reference voltage being a function of the supply voltage. The complimentary ones perform much better due keeping the current through the reference element constant. Actually this configuration is formed by two CCS that have each others reference voltage element connected to the output.
I guess that the poor HF PSRR of the complimentary CSS has to do with two CCS in parallel having higher capacitance.
The step response reveals the instability of the ANF current source while all other types do not show any overshoot. The stability of the ANF CCS can be improved, but this will affect performance unless further measures are implemented.
The output impedance is lowest for the simple and also the complimentary constant current sources using LEDs as reference. The CCS using a Zener diode as reference has roughly three times higher output impedance. Obviously cascoding the CCS helps to increase the output impedance considerably as the complimentary CCS with cascode shows. Clearly, the ANF type CCS has by far the highest output impedance.
The ANF CCS has shown some compelling properties, but also some issues to deal with. Michael Kiwanuka has written the most exhaustive paper about this type of current source that I have ever seen. On 15 pages he explains every detail including how to stabilize this type of CCS. This paper was super difficult to find. I copied it here four your convenience:
This investigation has confirmed that different current sources behave very differently. There will likely even be an impact on sound due to temperature dependent bias drift of some stages of the amplifier. In case of the high power amplifier I built in year 2010, the current sources powering the input stage also bias the whole amplifier up to the power output stage. For this project I chose the ANF CCS due to the added safety that comes as a side effect of the strong negative temperature coefficient, resulting in lower OPS bias at higher temperature. The tendency of the CCS to overshoot on transients may also influence the sound. My personal favorite is the complimentary CCS with cascodes in case there is sufficient voltage headroom available. Second favorite is the complimentary CCS without cascodes in case there is only little voltage headroom available.
Audioxpress published a great article about constant current sources written by Walt Jung.