Explain the transconductance amplifier and its important parameter
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The application of the operational transconductance amplifier (OTA) in the design of simple amplifiers with voltage-controllable gain and to the design of firs-order and second-order active filters with controllable gains and controllable critical frequencies is demonstrated. A typical biasing scheme is also shown so that readers can more easily set up the circuits themselves.
Introduction
This paper demonstrates the usefulness of the operational transconductance amplifier (OTA) as a replacement for the conventional op-amp in both first and second-order active filters. It is at least partially intended to acquaint the technology student w ith the rudiments of operation of the OTA, as well as the practicalities of using the presently available commercial OTA’s.
The structure of this paper is as follows. First, the basic operation of the OTA, including DC and AC operation, is explained. Simple example circuits will be presented in this section to demonstrate the similarities and differences between circuits w hich use the conventional op-amp and the OTA. Second, OTA active filter circuits will be presented and analyzed. The richness of the filter possibilities inherent in the second-order structures will be demonstrated. Finally, the last section will presen t practical considerations which must be considered when using the present generation of OTA’s.
II. Basic OTA Operation
1. DC Operation
The OTA is a transconductance type device, which means that the input voltage controls an output current by means of the device transconductance, labeled gm. This makes the OTA a voltage-controlled current source (VCCS), which is in contrast to the conventional op-amp, which is a voltage-controlled voltage source (VCVS). What is important and useful about the OTA’s transconductance parameter is that it is controlled by an external current, the amplifier bias current, IABC , so that one obtains
Introduction
This paper demonstrates the usefulness of the operational transconductance amplifier (OTA) as a replacement for the conventional op-amp in both first and second-order active filters. It is at least partially intended to acquaint the technology student w ith the rudiments of operation of the OTA, as well as the practicalities of using the presently available commercial OTA’s.
The structure of this paper is as follows. First, the basic operation of the OTA, including DC and AC operation, is explained. Simple example circuits will be presented in this section to demonstrate the similarities and differences between circuits w hich use the conventional op-amp and the OTA. Second, OTA active filter circuits will be presented and analyzed. The richness of the filter possibilities inherent in the second-order structures will be demonstrated. Finally, the last section will presen t practical considerations which must be considered when using the present generation of OTA’s.
II. Basic OTA Operation
1. DC Operation
The OTA is a transconductance type device, which means that the input voltage controls an output current by means of the device transconductance, labeled gm. This makes the OTA a voltage-controlled current source (VCCS), which is in contrast to the conventional op-amp, which is a voltage-controlled voltage source (VCVS). What is important and useful about the OTA’s transconductance parameter is that it is controlled by an external current, the amplifier bias current, IABC , so that one obtains
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