Constant current sources for audio amplifiers

Abstract

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.

Circuits

In general, both source and sink were investigated. For simplicity, only the schematic of the source is shown.

LED reference CCS

LED reference constant current source
This is a very simple current source using a LED as reference. This is one of the most common constant current sources seen in many audio amplifiers. The LED mostly compensates the transistors temperature coefficient, resulting in low thermal drift of the output current. Resistor R1 sets the current for the voltage reference and R2 sets the output current.

Zener reference CCS

Zener diode reference constant current source
This CCS is just like the one with the LED as reference, but uses a Zener diode (D2) instead. This results in much higher voltage headroom required for operation. It is yet to see whether it has any advantages over the CCS using a LED as reference voltage. Resistor R3 sets the current for the voltage reference and R4 sets the output current.

Amplified negative feedback CCS

amplified negative feedback constant current source
This CCS uses a transistor (Q4) as voltage reference. The reference voltage is not only a fuction of the current set by resistor R5, but also a function of the current through resistor R6. This way, the reference voltage forms a feedback loop together with Q3. Just like any other circuits that has negative feedback, this one should have better performance, but also has stability issues that need to be mitigated as shown later. In theory, this CCS could show little dependence on temperature in case the two transistors are thermally coupled as they compensate each others temperature coefficient.

Complimentary CCS

complimentary constant current source
This CSS is formed by two complimentary CCS that feed their output current through the reference path of the other CCS. This way a feedback loop is formed, but without amplification. The part count is twice as high as the normal LED reference CCS presented earlier. The reference voltage could be derived by different methods, but the LED is my preferred reference due to low voltage headroom requirement and low thermal drift. Resistors R7 and R8 set the output current and need to be of same value.

Complimentary CCS with cascodes

complimentary constant current source with cascodes
In case the CCS is used in an application with a large voltage across the CCS, cascoding the CCS may be a good option. The early effect of the control transistor is reduced and power dissipation shifted towards the cascode transistor, which allows to use fast and small signal transistors for the control transistors. Voltage headroom requirements of cascoded CCS is a bit higher in general and part count is also increased significantly. Resistors R9 and R10 set the output current and need to be of same value. LEDs D5 and D6 are voltage references. LEDs D7 and D8 provide voltage reference for the cascode transistors Q9 and Q10. LEDs as voltage references are convenient because the voltage is nearly the same regardless of current. In practical appcilactions, the current is constant and resistors instead of LEDs are cheaper and contribute to stability of the cascodes by increasing the impedance driving the cascodes.

Thermal stability

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.

Supply voltage dependency

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

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.

Step response

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.

Output impedance

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.

Amplified negative feedback CCS in detail

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:

ANF constant current source compensation
Following the findings in Michael' paper, I tried to set up a complimentary ANF CCS, which in theory would outperform all other CCS investigated so far, but I experienced difficulties taming the oscillation. It turned out that the oscillation didn't have to do with the complimentary design, but the ANF CCS itself. I didn't succeed to stabilize the ANF CCS as described by Michael. This may be having to do with unlucky setup of my simulation environment and likely it is possible to stabilize the ANF CCS properly. Anybody considering the ANF CCS is well advised to implement some measures for stabilization by all means. A stable current source is the foundation for a stable amplifier and I don't see a potentially unstable CCS as an advantage regardless of otherwise excellent properties.

Summary and conclusion

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.

Further reading

Audioxpress published a great article about constant current sources written by Walt Jung.