Logical Expressions - OTA1

About the writer: Harvey Morehouse is a contractor/consultant with many years of experience using circuit analysis programs. His primary activities are in Reliability, Safety, Testability and Circuit Analysis. He may be reached at harvey.annie@verizon.net. Simple questions for which I know the answer are free. Complex questions, especially where I am ignorant of the answers, are costly!!!

Summary: In previous articles of this series I showed how to create logical expressions. In this blurb I show how to prepare an Operational Transconductance Amplifier model useful with SMPS controller chips.

OTA1:

An OTA is a voltage-to-current amplifier. Its operation is similar to that of a vacuum tube, or a voltage to current controlled source. It is not a popular device, but several interesting devices are OTA devices and this device is also used within many interesting chips. While one could use a voltage to current controlled source with realizations, a unique device with its own symbol highlights the design difference of this device. Several OTA implementations have another input. At a later time this will be incorporated as an OTA2 device type.

OTA1 Model :

Figure 1 following shows the OTA1 device model, together with its symbol.

Figure 1
OTA type 1 circuit model

Source B1 has a transconductance value which is passed to the parameterized subcircuit, as are the values of R2 and C1. R2 and C1 establish the AC characteristics of the device.

A test circuit for this device is shown in Figure 2 following:

Figure 2
OTA type 1 test circuit

In Figure 2 two instances of an OTA type 1 are shown, one with the circuit implementation in a test configuration and the second with a parameterized subcircuit model. A graph of the output of this device with an AC frequency sweep is shown in Figure 3 following:

Figure 3
OTA type 1 test circuit

Clearly from Figure 3 the output voltages in DB and their phase, for out2 and out3 coincide. The circuit is so simple no netlist is required.

Summary:

An OTA circuit, type 1, has been prepared which may be useful in some simulations. Some precautions must however be used. The circuit is meant to be used in a closed loop configuration, as with large differential voltage inputs it will not accurately represent the operation of the device. This occurs because the real device output voltage and currents are limited, unlike that of the model. However for many applications this device is well suited to represent an OTA type 1.