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As you all know my immense interest in vacuum tubes, I`d like to discuss the most important part of the Push Pull tube amplifiers and that`s the phase splitter (inverter). All examples here use MOSFETS, because these transistors closely resemble tubes in operating characteristic (especially triodes).Since tubes don`t come in complementary pairs like transistors, in order to drive them in a push pull fashion, one must first amplify the signal to an appropriate driving level and then feed it to the two tubes in anti-phase. As one tube increases the conduction by a given amount the other tube will decrease its conduction by the same amount and vice versa. When complete balance is obtained (and that`s hard for several reasons, which ill explain further on) both tubes will conduct symmetrically and in theory all even order distortion should be cancelled out in the output transformer and all odd distortion summed. This is why, despite that push pull amps often work in class B, they tend to obtain much better distortion to signal ratios than single ended amps. So the job of a phase splitter is to feed the two output tubes with an antiphase signal in respect to each other. This can be done in several ways, a couple of which i`ve included here.
The first phase splitter is basically a differential amplifier, but is also knows as a long tailed AC coupled phase splitter. The biggest advantage is that it usually has some voltage gain, and equal output impedances, which, when properly designed, will give a very good balance. It`s comprised of two stages. The first one works as a common cathode voltage amplifier, nothing fancy actually. The second stage works as a common grid amplifier, for which the input is the cathode. So if we join the two cathodes and properly construct the bias, we should get perfect balance, right? Well it`s slightly more difficult than that. In order to get perfect symmetry, there should be no AC leakage from cathode to ground (e.g. the input of the first stage should be the same for the input of the second stage, not dampened at all). This can easily be obtained by using a constant current sink in the cathode chain to present a high impedance for AC and controlled resistance for DC. The advantage of some voltage gain comes with yet another advantage and that is that both outputs have the same output impedance, so if loaded evenly there should be no mismatch between them. The disadvantage is apparent, that you have to use two amplifying devices to handle the phase shifting. This is the most preferred design for Hi-Fi systems as it gives best balance and some voltage gain, minimising the need for other preamp stages. The distortion given here, at perfect balance is 0.0001%.
The second example is commonly referred as a Cathodyne, which is actually a splitt load amplifier. The advantage of this type of phase splitters is that when loaded evenly, the balance should be perfect, and it uses just one amplifying device as opposed to other methods. Unfortunately there are a few disadvantages that can`t be neglected. If any output load imbalance occurs, which happens often when output tubes are overdriven (because the grid starts drawing current), the good symmetry it gives when balanced, goes to the crapper. Also it gives less headroom for the signal swing compared to other methods. It gives roughly 1/4 Vcc for the signal swing as the supplying voltage is distributed evenly among the loads. It has a voltage gain of slightly less than unity (around 0.95-0.98) and needs a voltage preamp stage to obtain decent signal levels. Another disadvantage is that it`s not very stable and can cause high frequency oscillations in the output transformer. This is brought by the fact that the cathode may be bypassed to ground by a parasitic capacitance of a few pF. For a cathode load of 68k and 40pF the frequency at which the anode starts boosting the amplification is around 60KHz, well within range of a typical output transformer`s self resonance frequency. Trust me, you don`t wanna go there! It does have one more advantage though. The input is bootstrapped and a 1Mohm grid resistor may have around 30Mohm AC impedance, so in any case the previous stage will not see any load as an output when driving this phase splitter. These phase splitters are used mostly in guitar amps, where distortion is not a factor, and is actually beneficial, especially in overdrive conditions.
There are a few other phase splitter topologies, but they`re a derivative of these two, an frankly I don`t have the workspace to include them. So yeah, enjoy :)
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