I have seen in some books, in the transformer-coupled Class A amplifiers

  1. The transformer is connected between the collector and the VCC.
  2. The transformer is connected between the emitter and GND (emitter follower configuration).

If use the emitter follower configuration, we can get a high input swing and for the collector connected version, we can obtain a voltage gain.

Now, my question is what is the best practice or the correct way?

  • 1
    \$\begingroup\$ There is no one best practice. Different topologies offer different advantages and disadvantages; different problems require different solutions, and different designers have different preferences. \$\endgroup\$
    – TimWescott
    Oct 29, 2019 at 16:15
  • 1
    \$\begingroup\$ Both are correct and both cover different applications with some overlap. To decide you need to be specific about load impedances, power rail voltages, maximum operating maximum frequency, input drive levels, required output levels i.e. virtually everything that you haven't stated. However, the same applies if you don't have a transformer so go google why CE and CC have various circuit advantages/disadvantages. \$\endgroup\$
    – Andy aka
    Oct 29, 2019 at 16:16

1 Answer 1


Putting the transformer in the emitter (ie CC mode) is rare and has the disadvantage of requiring more drive power.

It is extremely unusual and not really a good example for a teaching book to use.

In the vast majority of cases, a push-pull configuration would be used in a power amplifier to improve efficiency and cancel the magnetic field created in the transformer as a result of the collector current. In a common-emitter configuration, the two collectors are at different AC potentials and require insulation from each other and the heat-sink. If the emitter follower arrangement is used they are at a common potential and so can be mounted directly to a common heat-sink thus reducing the thermal resistance of the system.

A big disadvantage is a requirement of a high-voltage drive to the base - the base will need to swing from slightly above the positive rail to far below the negative rail (assuming NPN devices). This complicates the driver circuit considerably. The only practical way to do this is to use a transformer from the driver to the output stage.

This arrangement was rare back in the early years of transistor power amplifiers - I would not expect any modern design to be lke this. One reason is that the relative cost of transformers and active devices has radically changed. Now transformers are expensive, transistors are virtually free. Whereas in the past transistors were the expensive item.

  • \$\begingroup\$ The emitter follower transformer-coupled amplifier is discussed in pg 591 of "Electronic Circuits, Analysis, and Design" by Prof. Neamen. However, I was unable to find any other resources for this whereas any google search results in the other configuration. \$\endgroup\$ Oct 29, 2019 at 16:53
  • \$\begingroup\$ If we set the power amplifier at the last stage to amplify a signal with full swing between VCC and GND, isnt the CC mode the best setting? \$\endgroup\$ Oct 29, 2019 at 16:56
  • \$\begingroup\$ Common collector mode is almost certainly the worst possible arrangement. \$\endgroup\$ Oct 29, 2019 at 17:56

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