Hybrids and Compounds

If two or more transistors are connected in a way that they emulate a single transistor with differing/improved characteristics, we speak of hybrids or Compounds.

Known examples are the cascode, the Darlington and the complementary follower, or Sziklay-pair.

 

Cascode

Cascodes
Cascodes

The cascode is a circuit in which a first driving transistor, the Master, is asisted by a second transistor, the Slave, wired in common-base/common-gate towards the supply line. As a result the Master´s collector-emitter voltage (or drain-source voltage) remains quite constant. It allow the usage of lowvoltage transistors with higher voltage rails.

You find cascodes as emitter-/source-followers in output stages of buffers and power amps. The output signal is hereby taken from the emitter/source of the Master transistor.

Of special importance is the omittance of the Miller-effect, which results in low input capacitances and large bandwidth.

This applies to amplifier input and voltage-gain (Vas) stages where the output signal is taken from the collector/drain of the Slave transistor.

 

The JFET-hybrid variant c) was very popular with japanese vintage amps of the 70s and 80s. Modern amps tend towards the bipolar cascode a).

The JFET-cascode after d) is specially well suited for line level buffers.

No additional parts are required for biasing purposes.

The Master JFET works under very constant and lowish drain-source voltage conditions, as the Slave JFET kind of ´rides´ the output voltage. Due to the constant voltage condition the Master´s collector-/drain-capacitance is not modulated, which results in low distortion.

It must be kept in mind thoug that the amount of drain-source voltage for the Slave JFET subtracts from the supply lines, hence the headroom and efficiency is smaller than for a single JFET-follower.

Darlington

Darlingtons
Darlingtons

In a Darlington a second Slave transistor is wired to the driving Master transistor´s emitter/source. Their current gains multiply. This allows for lower drive current demand and higher possible output currents.

The JFET- or MOSFET-hybrid requires only current while to charge the Master´s gate capacitance. Otherwise it can be driven without requiring drive power due to the extremely high gate input impedance.

The Darlington can mostly be found as output stage of power amplifiers, sometimes even in a triple configuration.

CFP, complementary follower pair, Sziklai-pair, Compound-pair

CFPs
CFPs

The CFP resembles the Darlington.

The obvious differences are the use of a complementary transistor type for the Slave and that it is wired to the collector/drain of the Master transistor.

The CFP can be found as output stages in power amplifiers, but by far not as common as the Darlington. It is a typical output stage in Rail-to-Rail OP-Amps.

It is also suitable in input stages and as buffers.

The advantage of the CFP is a the very short and fast current feedback loop from the Slave´s collector/drain to the Master´s emitter/source.

The large signal behaviour improves considerably.

The temperature sensitivity of the bias current when used as power amp output stage reduces and the optimum bias current when used in class-AB is is a lot lower. Headroom and efficiency are increased.

Bandwidth is lower though and its inherently prone to oscillation and may require additional C- or RC-compensation for stability