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The information presented in this cross reference is based on TOSHIBA's selection criteria and should be treated as a suggestion only. Please carefully review the latest versions of all relevant information on the TOSHIBA products, including without limitation data sheets and validate all operating parameters of the TOSHIBA products to ensure that the suggested TOSHIBA products are truly compatible with your design and application.
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Is there any way to amplify a signal with a voltage close to the power supply level?
In order to amplify a signal with a voltage close to the power supply level, use a rail-to-rail op-amp instead of a typical op-amp. The correct operation of a non-rail-to-rail op-amp is not guaranteed when the input is outside the common-mode input voltage range and close to VDD.
The differential input pair of a typical op-amp is composed of P-channel MOSFETs as shown in the following equivalent input circuit.
It is necessary to operate all the MOSFETs of an op-amp in the saturation region in order to prevent unwanted output distortion etc. Therefore, the following equation must be satisfied:
|VDS| > |VGS| - |Vth|
Qp1, which acts as a current source, enters the linear region when the input voltage is between VDD and (VSD of the circuit’s current source) + (VSG of the differential input pair). An op-amp enters the linear region when an input voltage within (VSD of the circuit’s current source) + (VSG of the differential input pair) is below the supply voltage. This voltage range is out of the common-mode input range. An input voltage in this range results in an output distortion or a reduction in gain. It is therefore necessary to operate op-amps in the common-mode input voltage range.
The differential input pair of a rail-to-rail op-amp is composed of a pair of differential P-channel MOSFETs and a pair of differential N-channel MOSFETs connected in parallel. Therefore, a rail-to-rail op-amp can be used with an input voltage between GND and VDD.
Related Links
The following documents also contain related information:
- Parametric Search : Operational Amplifier ICs and Comparator ICs
- Parametric Search : Input and output full range
- FAQs : What is the purpose of using a differential amplifier such as an op amp? (Common-mode rejection ratio: CMRR)
- FAQs : Are there any considerations for using an op-amp at low voltage?
- FAQs : What is the common-mode input voltage (CMVIN) of an op-amp?
- FAQs : What does rail-to-rail mean (Rail-to-Rail Op amp) ?
- Application Notes : Basics of Operational Amplifiers and Comparators (PDF:1.08MB)
- Mini catalog : Introduction to Op amps Comparators (PDF:600KB)
