Our Basic Circuits

 

Grounded Cathode Amp

Grounded Cathode Amp

The most basic building block of all circuit design, this has been in use, in one form or another, for over 100 years now. Useful for both voltage and power amplification, with excellent linearity. Further improvement in performance may be obtained with cathode degeneration at the expense of voltage gain, or by the use of an active plate load. With triodes, the linearity improves as the plate load increases. A Constant Current Source offers a much greater effective impedance (theoretically infinite, but certainly well in excess of a megohm in practice).

Ci and Co are used for DC isolation. These capacitors may or may not be required, depending on the nature of the source and load. Direct coupling, supply voltage permitting, is preferred as this eliminates a possible source of phase shift. RG is the grid DC return. Its primary function is to reference the control grid to DC ground. Since the grid draws no significant current, RG can be made quite large (up to 1MEG) to increase the input impedance of the stage. RP is the passive plate load. For triodes, linearity improves as this is made larger. It also serves to establish the stage gain. RK provides the Q-Point bias. Since the grid is at DC ground, raising the cathode voltage above ground makes VGK negative, which is what you need to set the no-signal plate current. CK bypasses RK to eliminate the cathode degeneration that would otherwise reduce the gain. Sometimes, eliminating the cathode bypass is done when there is more gain than needed, and/or to help linearize the plate characteristic.

The advantages are high voltage gain, high Zi, and current gain. Due to "Miller Effect", the input capacitance looks higher than it otherwise would seem from device characteristics. The effect increases with voltage gain, and can sometimes pose problems, especially when used with high resistance volume controls. This gives the Grounded Cathode amp the worst high frequency performance.

Characteristics

Cathode Follower

Cathode Follower

Cathode followers make excellent grid drivers for audio power finals. The circuit is sonically benign, having very little distortion of its own. Unlike the grounded cathode amp, the cathode follower is more capable of driving current to charge input capacitances, and to supply the needed grid current demand during fast transients that occur in all music programs. The cathode follower also has excellent high frequency performance, since there is no Miller Effect to increase input capacitance, which primarily is the reverse transfer capacitance between the grid and plate, since the plate is at AC ground. The CGK has very little current flowing through it since the grid and cathode are at nearly the same potential at all times, and its effective AC value is much lower than its static value.

Much of the reputed poor sonic performance of the cathode follower is due to its misuse. It offers a very low output impedance, but only to a high impedance load. It can not drive low impedances very well.

The solid state equivalent is the source follower, implemented with power MOSFETs. The MOSFET, having much higher gain than any vacuum tube, offers a much lower output impedance. The very small rdon allows for greater current sourcing as well. These characteristics are what is needed to drive difficult loads, such as finals operating in Class A*2, or audio power triodes, as these tend to draw grid current even before Vgk actually goes positive. Like its hollow state equivalent, the source follower is sonically benign. It offers much improvement over the traditional hollow state topologies: SRPP (a vacuum tube active pull up/pull down circuit) cathode followers, or interstage transformers.

The cathode follower doesn't produce any voltage gain since the cathode is fully degenerated. Its primary uses are as a buffer to isolate stages, and as an active pull-up circuit.

Characteristics

Differential Amp

Differential Amp

This is the differential amp, a.k.a. Long Tailed Pair. This two stage amp operates basically as a linear subtractor. It may also be used with balanced or unbalanced inputs and/or outputs. In this regard, it functions as a phase splitter for push-pull operation, converting an unbalanced input into balanced output. As a phase splitter, it offers voltage gain, has excellent AC phase balance, and unlike a good many other phase splitter circuits, it has the same harmonic distortion on each phase. Being a balanced circuit, it also cancels the even order harmonics. Operation can be improved significantly by providing an active tail load (Constant Current Source). The CCS tail load improves phase balance, and harmonic balance. This is much better than the usual solution: unbalanced plate resistors to force AC balance at the low frequencies. (This being necessary due to the low gain nature of triodes, and the fact that the input triode sees Vi - Vtail as its input, whereas the opposite triode sees just Vtail as its input.) Since the differential is a linear subtractor, it provides a convenient gNFB summing node.

Characteristics

Cascode

Cascode

This is another two stage amp, this being a cascade of a grounded cathode amp driving a grounded grid amp. The "traditional" use for the cascode has long been as a VHF voltage amp. The grounded grid amp has excellent high frequency performance since it, like the cathode follower, avoids CMiller. It does, however, have a very low input impedance, which is something you don't want to see very often. Letting this low impedance load down the grounded cathode stage kills most of its gain (ideally reduce the gain to unity or even less -- accomplished with transistors, but not tubes). The greatly reduced gain limits CMiller to small values that don't impact the high frequency performance nearly as severely as it would with a grounded cathode stage operating at the higher voltage gains. The cascode is one of those rare cases where you can have your cake and eat it too: the high frequency performance of the grounded grid amp, with the high input impedance of the grounded cathode amp. Being a two stage amp, it also provides significantly more gain than that of a single triode (theoretically up to u2, but more practically, 4 -- 5 X u).

It has long been considered that the two stage combination makes a faux tetrode: that's where the name comes from -- a combination of "cascade" and "tetrode". In an great many ways, it does tend to operate much like a tetrode, minus the screen grid "kinks", the screen current and its inevitable partition noise. The grid of the upper triode is still negative with respect to its cathode, so does not pull current. As an audio amp, it still retains much of the sonic signature of triodes, but with enhanced linearity. Since the full output voltage swing is occurring across two triodes in series, each triode sees less Vpk swing than a single triode would, thus they operate over a narrower range of voltages, and don't stray as far into nonlinear territory. Tube cascodes are rarely seen in hollow state audio practice, though they are used extensively in solid state amps.

When combined with the LTP phase splitter, the extra voltage gain often allows the elimination of an extra gain stage while preserving enough gain margin for good input sensitivity even under gNFB. This, too, is an advantage in that it gets one more active device (and its distortion) out of the signal path. Though not often seen in audio equipment, the cascode LTP has long been used in precision, wideband applications, such as oscilloscope vertical deflection amps. When connected as an LTP phase splitter, AC balance is improved by using a CCS as the tail load.

If there is a downside, it's the much greater output impedance. This may not play well with the input capacitance of a subsequent stage, resulting in premature roll off at the upper end of the audio band. This can, however, be helped by connecting to a cathode follower, with its much reduced input capacitance.

The AC coupled version is known as a "folded cascode", useful when you don't have the supply voltage reserve. Otherwise, the operation is identical to that of the direct coupled version seen here.

Characteristics


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