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When we build linear power amplifiers, we always need to choose some device for the output stage. Each has its own strengths and weaknesses which forces us to choose between them.


If, perchance, we wanted to build a very simple and accurate amplifier, we can safely ignore valves, since they all need heating circuitry and are not simple for a true hi-fi amplifier.

That it is possible to build a valve 2dj201 to a high specification is not in doubt, but they tend to be complex and expensive. BJTs are often used, but they do not respond well to even momentary overloads. This is because they suffer from second-breakdown – an instantaneous and catastrophic failure mode. They still need thermal compensation and a suitable gate drive design, and can suffer from a ‘latch-up’ condition in some cases.

Datwsheet there is the MOSFET, which does not suffer any second-breakdown effects although this is not strictly true – see below for more info. These devices are extremely rugged, yet they do have a large nonlinear gate capacitance to deal with. datssheet

Using HEXFETs in High Fidelity Audio

If driven incorrectly, they show high distortion levels, especially vertical types – most commonly these days, HEXFETs. The update below has some important information that I recommend you read thoroughly and make sure you understand before settling on the use of HEXFETs in your next amp project.

While there appear to be many advantages to their use over BJTs, HEXFETs may often suffer from exactly the same problems – thermal runaway and a failure mode that is suspiciously similar to second breakdown. On top of this, there is a much larger voltage loss This voltage is usually taken from the main supplies, daasheet for a given supply voltage, expect a little less output power.

Well, that is the easy route to take for designing an output stage. Any device can be used for audio and give great performance if a proper design is found.

It is just easier to use more linear devices. When found, they tend to be quite expensive. This article is intended for Class-AB designs. If you are designing a class A amplifier, the first trick see below should be used the second is not needed since the bias is already quite high.

Alright, now for some explanations. There is not only one capacitance to deal with, but two one from the gate to source and the other from the gate to drain. This is the main problem: Through a lot of time and molten breadboards, I found the best two things to design for are the following:.

For the design of the amplifier, I will assume a single LTP input stage. Better performance can be seen by using multiple LTPs, but this will not be a simple design in fact it will be quite complex with high frequency stability issues needing attention.

If we look at these complementary device data sheets, we will see very different figures for current capability, on resistance, and, most importantly, gain or forward transconductance. But if we use a matching tool, we will find that the gain varies considerably from the actual devices vs.

That is why we need to buy a few extra and match them together. Also take note that HEXFETs will require a Vbe multiplier for thermal compensation, since the negative temperature coefficient does not come into play until the device has about 10 amps through it at least for the IRFP The exact values around the Vbe multiplier also known as a bias servo are critical to ensure that the thermal performance is matched as closely as possible.


In every practical design I have tried I had to use a class AB driver stage. A class-A driver will work fine if you really want an electric heater, as you will see in the next calculation. Now, in order to size-up the proper driver for the FETs, we need to do a little maths. I promise it is not hard. An example would work nicely here For a better understanding, a simplified output stage circuit is shown below.

We will do calculations using the gate charge method, which IR recommends AN Don’t add these yet! We will find each device’s needs individually. The general formula to determine gate current is The multiplication factor of gives the headroom needed for accurately reproducing a square wave or high frequency sinewavesince the gate driver needs a lot of current to quickly switch the MOSFET from OFF to ON.

Although the requirement for this is minimal the CD format is incapable of anything even approaching a square wave above a couple of kHzit has become an expectation that power amps should be able to reproduce perfect square waves at 10kHz as a minimum. Multiplying each figure by five because there are five devices of each polarity gives us 1. So a Class-A driver would need bias set to 3. The value for R7 will depend on the linearity of the driver transistors.

I had to guess and check with my ammeter to get a good value. This can range anywhere from Ohms up to perhaps 5k. Make sure you check the idle current before calling the design done! These drivers Q7 and Q8 may need a heatsink.

Also note the capacitor in parallel with R7. This should be of a high value i. These current figures seem quite high, but keep in mind this current will only last a very short time compared to the signal, and virtually no current is needed to keep the devices either in the OFF or ON state. The current to reproduce a sinewave will be a bit lower, since it is a smooth curve, but this much headroom will drastically lower distortion.

This is why we cannot practically use a class A driver, unless, of course we use one pair of output devices. The large notch is at the second harmonic, and the small bump to the right is the fourth harmonic. Barely any third harmonic is seen. Not very good for a true hi fi, unless we are making a valve-like amplifier. Even this will not show the same effects as a true valve amp – the nature of the distortion components will almost certainly be different.

Adjusting the bias to 1 amp removes nearly all distortion, yet now we are approaching a heater I mean class A. As the xatasheet shows, the second harmonic was reduced considerably, while the fourth harmonic is below the noise floor. This greatly improved the dqtasheet. At one watt, the distortion is not measurable at all, unlike with the class A driver. Reducing the gate resistors to 4. There is no evidence of ‘notch’ distortion or any other nasty odd harmonic, only a ‘nice’ second harmonic added in.

Also note that this amp was built on a breadboard. A compact and nicely wired PCB should decrease distortion even further.

It looks very simple, and includes the class AB driving stage to improve gate driving. It’s very simple compared to amps with multiple LTP stages. The minimum stability network Zobel shown is always needed, and a series inductor with parallel resistor may also be required. The values of these components will be found by experiment.

They are much more rugged than BJTs as my burned parts pile shows, and sound very good when a class AB driver is added. I hope this short article with aid others in using these ‘switching’ and ‘not linear enough for audio’ devices to get distortion figures below many good amplifiers with ‘very linear’ devices.


And remember one thing – any output device can be precise if a proper design is found. Finding the correct design parameters becomes more complex with non-linear devices. And many pages from ESP.

LM4702+2SK1530+2SJ201 Stero Power Amplifier PCB Board 100+100W

The above article is a contribution from Mitch Hodges, and ESP has not verified all aspects of the design process described. While the circuit can be and has been simulated quite readily with good results, this is no guarantee adtasheet everything will work as expected.

It may be possible to select the zeners to achieve basic current limiting, giving the amp some protection from overload conditions. Also, remember that a series inductor may also be required. It will be noted that there are no component values supplied – this is quite deliberate, and is not an omission. The article is intended to describe the design process and how to work around the inherent non-linearity of vertical Dxtasheet, and is not intended to be a construction project.

Requests for component values will not be fulfilled. It is probable that the constructor will be forced to compromise, using a significantly lower quiescent current than suggested just to maintain a sensible heatsink size and temperature.

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Reducing your expectations of the maximum frequency that needs to be passed at full power will reduce the loading on the Class-AB drivers, but does nothing for the MOSFET low current linearity. Compromise will be almost essential IMO.

Finally, I’d like to thank Mitch for his contribution, since it describes the issues and how to solve them in an easy to follow manner, keeping complexity to the absolute minimum in the final design example. While it worked well enough, giving the expected power output and with fairly low distortion, as noted above the required bias current is quite high to reduce crossover distortion to an acceptable figure.

As it transpires, the design I was looking at was unsuitable for the intended purpose, because the quiescent current needed to remove crossover distortion was too high to be practical.

In many cases, the lowest heat output possible is highly desirable, and HEXFETs are simply unsuited to any application where very low Iq is desirable or necessary. Bias stability is definitely an issue as discussed above. It is commonly and erroneously stated that MOSFETs are ‘safe’ because they have a positive temperature coefficient, so as a device gets hotter, its drain-source R DS on resistance increases.

This much is true, but this alleged ‘benefit’ is actually completely useless in a linear circuit. It can also cause major problems in switching circuits, but that’s another topic altogether and will not be covered here. What is not commonly noted is that all MOSFET devices have a fairly high negative temperature coefficient for the gate-source threshold voltage V th.

At the gate-source voltages needed to obtain typical bias currents, even a small temperature increase causes a large drain-source current increase, so the use of a carefully designed bias servo Q5, R5 and R6 in the Figure 4 schematic is absolutely essential. This point is made above, but is sufficiently important that repetition will not go astray.

To illustrate this, Figure 5 shows the graph from the IRF data sheet, and although it does not continue down to the levels we are interested in, the trend is clearly visible.