=20 =20

Virtual Ground Circuits

A common problem in analog electronics is having a requirement for a dual-voltage supply (e.g. =C2=B15=E2=80=89V) but only having a=20 single supply available, such as a battery. There are many ways to = =E2=80=9Csplit=E2=80=9D a=20 single supply so that it behaves like a dual supply. This article = describes=20 several such circuits and the tradeoffs involved.

This article is written with solid-state headphone audio = circuits in=20 mind. Generalizing this to other situations is an exercise left to the=20 reader.

Two Batteries

The simplest way to solve the problem of needing a dual supply when = using=20 batteries is to simply use two batteries in this configuration:

The problem with this is that if one battery drains faster than the = other=20 such that one gets down to about 1=E2=80=89V or lower=20 before the other gets low, the DC offset at the output will begin to = rise. (I=E2=80=99ve=20 tested this with several different op-amps. It=E2=80=99s possible some = designs won=E2=80=99t=20 have this problem.)

Batteries can drain unevenly for a number of reasons. Perhaps you = put your=20 batteries in a drawer after buying them and pull them out randomly and = draw an=20 old one and a new one. Perhaps you=E2=80=99re using rechargeables, and = one or more cells=20 is dying. Perhaps you=E2=80=99re just unlucky today.

To be fair, before this happens you will get a warning: the amp will = start=20 sounding bad for other reasons. It will probably audibly clip the music = well=20 before this danger point due to insufficient supply voltage, and it may = also be=20 current-starved due to battery exhaustion. So, the most likely way this = problem=20 will occur is if you leave a battery-powered headphone amp on for an = extended=20 period without music, or without listening to the music that is playing. = Ever=20 fall asleep while listening to headphones?

If you don=E2=80=99t turn the amp off before you get to this danger = point, the=20 resulting high DC offset is likely to damage your headphones. So, we try = various=20 virtual ground schemes to let us use a single battery and still have a = dual=20 supply.

Resistor Divider

The CMoy=20 pocket amplifier power supply is a resistor divider type virtual = ground=20 supply:

The two 4.7=E2=80=89k=CE=A9 resistors = create a =E2=80=9Cvirtual=20 ground.=E2=80=9D Let=E2=80=99s say there=E2=80=99s 12=E2=80=89V across this=20 circuit. The resistors are an 0.5=C3=97 resistive divider: there is = 6=E2=80=89V=20 at the midpoint of the divider. The =E2=80=9Cdistance=E2=80=9D between = the midpoint of the=20 divider and the negative side of the power supply is -6=E2=80=89V and the distance to the positive = side of the power=20 supply is +6=E2=80=89V. = Voil=C3=A1, two equal but=20 opposite voltages from a single power supply!

Unfortunately, this simple configuration is prone to becoming = unbalanced. To=20 see why, consider this schematic, a CMoy pocket amplifier driving = headphones,=20 drawn from the DC perspective:

The 1=E2=80=89mV battery (Vos) simulates the op-amp=E2=80=99s input = offset voltage. This=20 is a reasonable value for an OPA132, though it does vary between chips = in=20 practice.

This offset forces 1=E2=80=89mV across = R3. Because=20 op-amps always force their input voltages to be equal, this in turn = forces 10=E2=80=89mV across R4. As you can see, this = puts 11=E2=80=89mV=20 of DC across the load. If the load is 32=E2=80=89=CE=A9 at DC=20 (such as a pair of Grado=20 SR-60s), 0.34=E2=80=89mA is forced = through the load.=20 This current can only come from the rail splitter, which looks like two = parallel=20 resistors to the load. Ohm=E2=80=99s law tells us that since the current = is 0.34=E2=80=89mA=20 and the resistance is 2.35=E2=80=89k=CE=A9 (two 4.7=E2=80=89k=CE=A9=20 resistors in parallel), the voltage at the midpoint of the divider is = forced=20 ~0.8=E2=80=89V away from the ideal = midpoint.

In this particular situation, then, a 9=E2=80=89V=20 battery would split to about +3.7=E2=80=89V and -5.3=E2=80=89V instead of the ideal =C2=B14.5=E2=80=89V. Different op-amps, = headphones, and resistor=20 values will give a different split. Therefore, it is best to simply = realize that this offset will be significant with low-impedance loads, and it = will=20 increase as the load impedance goes down, rather than calculating offset = and=20 trying to counteract it somehow.

The Problem with Unequal Virtual Ground Splits

In a circuit like the CMoy pocket amplifier, an uneven virtual ground = split=20 doesn=E2=80=99t hurt the sound all by itself. The input and output are = both referenced=20 to the same ground point, so the shift doesn=E2=80=99t create an = electrical=20 compatibility problem. You=E2=80=99re probably asking, then, why worry = about it?

Most op-amps can=E2=80=99t swing the output voltage from rail to = rail; they have some=20 minimum distance. The OPA132, for example, needs approximately 3=E2=80=89V=20 of distance between the power rails and the output with relatively = low-impedance=20 loads like headphones.

Let=E2=80=99s say we=E2=80=99re using a 9=E2=80=89V battery, and under=20 load our virtual ground circuit splits that unevenly to +4=E2=80=89V and -5=E2=80=89V. Let=E2=80=99s also=20 say our output signal=E2=80=99s peaks are 1=E2=80=89V from ground.=20 Add in the 3=E2=80=89V of headroom needed = by the op-amp,=20 and we=E2=80=99re right at the clipping point on the V+ rail. Since our = power supply is=20 a battery, its voltage will drop over time, so we=E2=80=99ll get very = little run time=20 before it starts clipping.

Ways to Fix the Problem

One quick and dirty way to fix this problem is to simply increase the = power=20 supply voltage. But, this requires a larger, more expensive power supply = if=20 you=E2=80=99re using wall power, or more batteries.

Another way to fix the problem is to lower the virtual ground = resistors=E2=80=99=20 values. The problem with this is that it increases the current the = divider=20 draws. This is a balancing act: if the the extra current drawn from the = battery=20 is high enough, it can wipe out the run-time increase you get from = having a=20 lower battery voltage where clipping starts.

Most of the subsequent circuits in this article use an entirely = different=20 solution: buffering the virtual ground. These techniques make the = voltage=20 divider appear to have a very low impedance while still drawing little = current.=20 This keeps the virtual ground point nicely centered between the rails = under=20 load. The extra parts can easily pay for themselves by allowing you to = use a=20 smaller power supply, or by increasing your battery=E2=80=99s run = time.

Simple Buffered Virtual Ground Circuits

The most elegant buffered virtual ground circuit is Texas = Instruments=E2=80=99=20 TLE2426. This part is called a =E2=80=9Crail splitter:=E2=80=9D it = splits a single supply in=20 two, so you have two =E2=80=9Cvoltage rails=E2=80=9D plus ground. = It=E2=80=99s basically a glorified=20 voltage divider, so it replaces the resistors in the simple = resistor-divider=20 power supply: you apply a voltage between its IN and COM pins, and it = puts out =C2=BD=20 that on the OUT pin. Unlike a simple resistor divider, though, it has = some=20 buffering circuitry inside so it doesn=E2=80=99t become unbalanced. (Oh, = there may be a=20 tenth of a volt of error or so, but that=E2=80=99s a small matter.) = Here=E2=80=99s the modified power circuit:

The first schematic shows the simple 3-pin package, and the second = shows the=20 circuit for the 8-pin versions which have a noise reduction pin. The = latter has=20 slightly better performance.

Notice that there is just one capacitor across the battery instead of = a cap=20 between each rail and virtual ground as in the resistor divider supply. = In the=20 resistor divider circuit, two capacitors are absolutely necessary to the = success=20 of the circuit. Below, I will talk about the advantages of using two = caps like=20 this in an active virtual ground circuit, as well as the disadvantages. = For now,=20 assume that it=E2=80=99s better to have just one before the active = =E2=80=9Crail splitter.=E2=80=9D

The main problem with the TLE2426 is that it can only handle 20-40 mA = of=20 current, depending on conditions. If your load draws more than that, a=20 TLE2426-based power supply will become unbalanced. For higher-load = situations,=20 you can try a buffer-based power supply instead:

This is similar to the circuit inside a TLE2426. By making a rail = splitter=20 out of parts, we can get higher output current. Notice that the resistor = values=20 are much higher than in the simple CMoy power supply. By adding the = buffer, we=20 don=E2=80=99t need low divider resistors to keep offset under control. = Because the=20 resistor values are so high, the quiescent current of the circuit is = dominated=20 by the quiescent current of the buffer alone; the divider contributes = negligible=20 current draw.

The high resistor values work as long as the power draw on this = circuit is=20 evenly balanced, as it is in a simple headphone amp. If you have an = unbalanced=20 draw, the divider is likely to become unbalanced. In that case, you can = replace=20 the divider with a TLE2426. Another virtue of the TLE2426 over resistors = is that=20 it takes less space, and you don=E2=80=99t need to do resistor matching = to get high=20 accuracy. That=E2=80=99s how we did the virtual ground in the META42=20 amp.

I=E2=80=99ve used Burr-Brown=E2=80=99s BUF634 here. It can handle up = to 150 mA in the DIP-8=20 package, and in the larger metal-based packages it can source up to 250 = mA, with=20 appropriate heat sinking. There are many other open-loop buffers on the = market=20 that will give similar performance in this circuit. The disadvantages = relative=20 to the lone TLE2426 are that it=E2=80=99s more complicated, it costs = more, it has higher=20 output impedance, and it has a higher quiescent current draw (~1.5 mA = vs. ~0.3 mA).

If you can=E2=80=99t get a TLE2426 and you don=E2=80=99t want to mail = order one, this is a=20 closer substitute than the above circuit:

You can use a cheap generic op-amp =E2=80=94 such as the ubiquitous = =CE=BCA741 =E2=80=94 here.=20 It=E2=80=99s acting like a buffer, just as in the previous circuit. The = major difference=20 is that it has lower output current than the buffer, but unlike an = open-loop=20 buffer it has feedback so it has low output impedance. Low output = impedance has=20 many salutary effects on the circuit; in a heapdhone amp, the biggest is = lower crosstalk.

The 1=E2=80=89k=CE=A9 resistor in the = feedback loop is=20 arguably optional. Its purpose is to keep the op-amp stable in the face = of heavy=20 capacitive loads, such as bypass capacitors in the circuit being = powered.

If you use a cheap generic op-amp, this circuit=E2=80=99s performance = is no better=20 than for a TLE2426 and it takes more board space, so you should only do = that=20 when you can=E2=80=99t get a TLE2426. But, if you use a better op-amp, = you can get=20 better performance than a TLE2426. The main spec to look for here is = high output=20 current. More-or-less drop-in replacements with higher than average = output=20 current are the LMH6642 and the AD817.

The highest output current op-amps tend to be current feedback types. = These=20 require a bit more care in application than the common voltage feedback = type.=20 Consider this circuit, which can put out 250 mA:

C2 is the compensation capacitor, and R3 is there to reduce supply = current a=20 bit as explained in the datasheet in the section on the shutdown = feature.

If you need even more than 250 mA, the LT1206=E2=80=99s big brother, = the LT1210,=20 works in a very similar circuit. Other manufacturers make similar = high-current=20 CFB chips that can work here, but read their datasheets before making = circuits=20 for them: CFB op-amps generally won=E2=80=99t drop into an existing = circuit without=20 changes.

Another option is to make a buffer from generic discrete components. = This=20 simple design comes from miniaturization guru Sijosae:

The transistors can be most any complementary pair of small-signal transistors. Suitable alternatives are the PN2222A and PN2907A.

The diodes are generic small-signal types. An acceptable alternative = is the=20 1N914.

This circuit has better performance than a simple resistive divider = virtual=20 ground, and the parts cost is lower than for any other circuit mentioned = here.=20 It is, however, the least accurate of the buffered virtual ground = circuits.

Getting More Complicated

The above buffered virtual ground circuits have one of two major = problems.=20 The TLE2426 and VFB op-amp based circuits have fairly low output current = abilities. The other circuits have higher output current, but most lack = feedback=20 so their output impedance is relatively high; this can result in = problems like=20 increased crosstalk in a headphone amplifier. For simple circuits, the = CFB=20 circuit above is the best balance of high output current, low output = impedance,=20 and simplicity.

If you can sacrifice simplicity, you can still use VFB op-amps by = combining=20 them with a buffer, like so:

By wrapping a buffer in an op-amp=E2=80=99s feedback loop, you get = the higher current=20 ability of the buffer plus the high accuracy afforded by feedback.

The value of the resistor between the buffer and op-amp may need to = vary in=20 your circuit. If you=E2=80=99re getting peaking at high frequency or = even instability,=20 you need to raise its value, to perhaps 1=E2=80=89k=CE=A9.=20 Similarly, the compensation capactior CC might=20 need to be increased if you=E2=80=99re having instability problems; it = probably shouldn=E2=80=99t go much higher than 100pF.

You can replace the resistor divider with a TLE2426 to get some of = the=20 benefits described above. Then it=E2=80=99s just one small step from = there to the ground=20 channel concept used by the PIMETA and PPA amplifiers:

The ground channel concept works best when you have many small ground currents and one big one. In a headphone amp, the circuit has several = resistors=20 and such going to ground, but virtually all of the dynamic current to = ground is=20 the return current from the headphones. The buffered op-amp handles the = big=20 currents (OGND), and the TLE2426 sets the input of the big driver and = handles=20 all the small currents (VGND).

For audio, I prefer to use the same op-amp and buffer in the virtual = ground=20 as I do in the audio driver circuits. For instance, if the audio = channels use an=20 AD8610 op-amp and a HA3-5002 buffer, I will usually use those parts for = the=20 virtual ground driver as well. This gives the most symmetric performance = since=20 the virtual ground driver and the headphone driver circuits effectively = sit=20 across the load from each other.

Capacitors on the Output of a Virtual Ground Driver

Above I said that when moving to an active rail splitter, you want to seriously consider putting the rail capacitors in front of the = splitter. The=20 purpose of putting caps across the resistive ground divider shown at the = start=20 of this article is because this passive splitter cannot deliver very = much=20 current, so we need the caps to do that. The resistors are only = maintaining the=20 DC level of virtual ground here. An ideal virtual ground circuit would = have=20 infinite current delivery, so there should be no advantage to putting = caps on=20 its ouptut. In fact, it can be detrimental.

An active virtual ground circuit has some = =E2=80=9Cbandwidth:=E2=80=9D that is, it will be=20 effective over some range of frequencies. If you put capacitors across = its=20 output, that lowers its bandwidth: as frequency goes up, the capacitors = are =E2=80=9Cin=20 charge=E2=80=9D to a greater and greater extent. If the caps are large = enough, the=20 virtual ground circuit=E2=80=99s bandwidth is completely swamped. It = could end up being good for no more than maintaining the DC level of virtual = ground.

Output caps can be a good thing if the rail = splitter has=20 a fairly low output current limit. That=E2=80=99s the situation in the = MINT=20 amp, for example. The TLE2426 has an output current limit of between 20 = to 40=20 mA, depending on operating conditions. When it goes into current = limiting, its=20 output goes to the negative rail, which would make for a massive shift = in the=20 virtual ground point, so we cannot allow this to happen. A heavy = headphone load=20 could indeed exceed 20 mA, so putting caps on the ouptut of the TLE2426 = saves=20 the design. Although the TLE2426 has no effect at audio frequency, it = still has=20 benefits compared to a resistive voltage divider. First, its output = impedance is=20 much lower, so the virtual ground shift described above doesn=E2=80=99t = happen. Second,=20 it requires less operating current than the CMoy=E2=80=99s resistive = divider.

Another potential problem with big caps on the output of a virtual = ground=20 splitter has to do with stability. Some circuits will become very stable = in this=20 situation: no bandwidth and no gain, hence no oscillation. Most circuits = aren=E2=80=99t=20 made to cope with capacitive loads, however. They become less = stable when=20 driving a capacitive load. Study the datasheets for the ICs you will be = using. =20 Unless they specifically tout the fact that they can drive large = capacitive=20 loads, beware of using them in virtual ground circuits. Don=E2=80=99t = forget to consider the system bypass caps, if they go from each rail to virtual = ground:=20 there are many chips out there that will become unstable with less than = a=20 nanofarad of capacitance on their output, and the bypass capacitors will = count=20 against this. Ultimately, you will have to build real circuits and test = them=20 before you know whether a given chip can cope with the capacitive load = in your=20 setup.

There=E2=80=99s one more problem with putting caps on the output of = the virtual=20 ground circuit: it wastes capacitance. In a very real sense, two caps = across the=20 output of a virtual ground circuit are in series, so the total = capacitance is=20 cut in half. Also, you require one capacitor in front of the rail = splitter, but=20 two after it. This means putting the rail capacitance in front of the = splitter=20 is actually four times as efficient: you can have twice the effective=20 capacitance in half the board area, or four times the capacitance in the = same=20 board area.

References and Acknowledgements

The DC-perspective CMoy pocket amplifier schematic above and the = original=20 explanation of it is due to PRR of Headwize, in this post.

Many other interesting virtual ground ideas and discussions are in = the th= read containing PRR=E2=80=99s post.

Sijosae posted his discrete rail splitter idea in th= is thread.

Section 4.1.5 in Jerald Graeme=E2=80=99s Optimizing Op Amp = Performance was=20 useful in designing the VFB op-amp based splitter. This section concerns = running=20 op-amps into capacitive loads, which will often happen with a virtual = ground=20 driver.