Uses for this circuit are more numerous than just the one of converting balanced into single-ended. For example it could be used to buffer the output of a Cascode amplifier. The Cascode suffers from a virtually nonexistent PSRR, whatever is on the power supply connection, will appear at the output. Now if we connected the non-inverting input of the Broskie Cathode Follower to the power supply connection and the inverting  input to the plate of the top triode in the Cascode circuit, we will have scrubbed the power supply noise from the signal. Like a transformer, if only one half of the input signal is used, the gain will fall by half as well. Of course, the inputs could be switched if the inverted output of the Cascode was desired. Another example would be to use two Broskie Cathode Followers for the output of a balanced preamplifier. This way we could achieve a clean, low output impedance, balanced output.
   The topology of the Broskie Cathode Follower can be easily transposed to FETs for they are depletion mode devices like the vacuum tube. With the addition of appropriate biasing circuitry the topology can be implemented with transistors or MOSFETs.

microphone preamplifier. To a rough sketch of two directions he could travel is found in this month's Design Idea.

The Circular/Bridge Amplifier
   This circuit counts as a buffer in that offers a low output impedance and no voltage gain. As the last few issues of this journal have dealt with this circuit, we will not go over the how's and why's, but rather what to expect from this circuit as a buffer.

Circular/Bridge Amplifier

     The easy mistake to make is to assume that this circuit is just two Cathode Followers in series with each other and that as a consequence the output impedance would equal twice that of one Cathode Follower. Do not forgets that the power supplies are effectively dead shorts to AC signals. It helps to redraw the circuit so that the two power supplies are shown shorted.

   Connecting a 1 volt battery across the output terminals will shift one terminal +0.5 volts higher and the other -0.5 volts lower than at idle. The grids of both triodes have not moved and remain fixed at the negative bias voltage. Consequently, the tube whose cathode was forced 0.5 volts negative sees a +0.5 volt increase in its grid-to-cathode voltage and conducts more current.

   A sweet microphone preamplifier or phono head-amp could easily be made from four FET's and a few resistors. On the other hand, one of our readers, Rowan, has asked for some direction in building a tube and only tube based

< PREVIOUS

pg. 11

NEXT >