What is a Power Operational Amplifier?

A POA Power Op-Amps is a hybrid combination of an operational amplifier at the point of input with a power output stage.

Put simply: this SERVOWATT concept offers you more. Not only with its first stage, which amplifies or inverts, but also with the high-quality difference input stage which has the capability to carry out accurate analogue calculations such as addition, subtraction, integration and differentiation. As a result our amplifiers can be used in a wide variety of applications. Further, directly from the controlling procedure they can help process other voltages or currents for speed, power, torque, pressure, temperature, etc. from signal transmitters. The power output stages are provided in over-dimensioned format to make it possible to delivery 2 to 6-fold impulse currents – reliably, time after time.

The difference amplifier principle

In principle, an operational amplifier is a differential amplifier capable of increasing the differential voltage at the point of input such that, with the help of the circuit, the level of voltage at the output is just right for the voltage at the input between E- and E+ to actually becomes to zero! On delivery all units are unconnected without programming board. This is because it is not until they are connected that the amplifier function is defined. The main circuit examples are outlined on pages 22 and 23.

The most important characteristics of a Power Op-Amps are: DC accuracy, speed and stable operation. In our experience, it is not possible to optimise all characteristics in one and the same unit. For this reason there are also versions with special characteristics, such as for higher frequencies, which we customise to specific requirements.

See the 463-page book entitled "OP Amps for Everyone" by Texas Instruments on our home page at www.servowatt.de

  A negative example
Instability: from transient effects to continuous oscillation
  (especially with small amplifications under 10x)
Sensitivity to capacitive loads
Stable operation: a key criteria

At higher frequencies every amplification stage causes a phase shift of 90°. So even 2 amplification stages with 180° can make systems unstable, i.e. they will oscillate. Usually all integrated OP amplifiers, such as the 741, have more than 3 amplification stages so you need to compensate for this internally. If the appliance also has a booster output stage you are bound to run into stability problems. The amplifier oscillates so it has to be compensated for to adjust the frequency response or the phase at higher frequencies. We solved this stability problem as long ago as 1970 by developing a discreet, input amplifier, with only 2 stages which directly drives the output stage.

The power operational amplifier: a tried-and-tested solution. Our idea was simple, though if we’re honest, arriving at the final answer was not. But after many years’ intensive experimentation and development work we finally translated our idea into a unique device suited to a broad variety of applications. These amplifiers still have to be made with discrete parts so that many of the features you are looking for can be integrated simultaneously. For instance:

Long-term DC stability, in terms of µV
High level of internal amplification and linearity
Range of power output greater than 20 kHz,
  with special versions beyond 200KHz
Linear output stage totally free of interference
Can be wired as required
Capacitively loadable output
Continuous short circuit resistance at output
Ability to deliver high impulse currents over
  circuit above the continuous current


With a stable Power OpAmps and the appropriate circuit,
stable solution are can be produced for a broad variety of
control engineering applications without the use of other
auxiliary amplifiers!

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SERVOWATT Leistungselektronik GmbH, 70839 Gerlingen near Stuttgart / Germany
Phone +49 (0) 7156 - 24041, Fax +49 (0) 7156 - 29944, Email info [at] servowatt.de