op amp

Operational amplifiers

Types of operational amplifiers (bioelectric amplifiers have different gain values) Low-gain amplifiers (x1 to x10)

Used for buffering and impedance transformation between signal source and readout device

Applications are measurement of action potentials and other high-amplitude bioelectric events

Medium-gain amplifiers (x10 to x1000) Recording of ECG waveforms, muscle potentials etc.

High-gain amplifiers (x1000 up to x106 ) Sensitive measurements, like recording EEG (brain potentials)

Operational amplifiers

Circuit symbol of the operational amplifier Vout=Aol(Vin(+)-Vin(-))

Operational amplifiers

Behavior of op-amps Output voltage can be in range from negative to positive supply voltage

- Rail-to-rail ops allow widest voltage range (nearly up to supply voltage)- Normal op-amps have lower output voltage range

The (-) input produce an output signal that is 180 out of phase with the input signal

The (+) input produce an output signal that is in phase with the input signal

No current flows in to either input terminal of the op amp (infinity Input impedance )

Op amp with negative feedback works as an amplifier (the two input terminals are at the same voltage)

Op amp with positive or no feedback works as a comparator

Operational amplifiers

Attributes of ideal op-amps Open-loop Gain is infinite

No offset voltage

Input impedance is infinite (acts as an idea voltmeter)- bioelectric amp must have very high input impedance because all the bioelectric signal source exhibit a high source impedance

Output impedance is zero (acts as an idea voltage source)

Zero noise contribution

Bandwidth is infinite (no frequency-response limitations, no phase shift)

Basic amplifier configurations

Basic amplifier configurations Inverting amplifier or follower

Non-inverting amplifier or follower

Summing amplifier

Differential amplifier

Transimpedance amplifier (amplifies and converts input current to output voltage)

Inverting amplifier or follower

Inverting amplifier or follower

The input-output plot of an inverting amplifier (fig) Linearity over a limited range of Vin The op amp is saturated at 13V (further increase in Vin no change in

Vout)

Inverting amplifier

Error sources - Inverting amplifier

Fig. 7-4 shows detailled circuit of an inverting amplifier Bias currents Ib- and Ib+ and output load current Io Three types of internal resistance and capacitance

(1) Common-mode Rcm and Ccm, referring to internal ground Vee (2) Differential Rdiff and Cdiff between positive and negative input (3) output Ro

Internal ground reference Vee as middle of positive and negative supplyErrors through external components Rs creates a 0.5% gain error (from the ideal -1V/V), Rs becomes part of a

voltage divider with R1 at the input.-This small error can sum up in multiple staged amplifiers

Ro creates another gain error through voltage divider behavior with the load resistance of the following stage

- In this case Rl is large enough, so the influence from Ro isnt strong enough

Error sources - Inverting amplifier

Errors through internal components

Rcm (is parallel with R1) causes small errors, as it is usually > 1000M

Through Ccm (< 5pF) higher gain errors will be produced in higher frequencies (Rc=1/jc)

-Example: at 1 Mhz Ccm reactance is at 32k, which shunts the external resistance, therefore creating a higher gain error

Other errors

Bias current Ib- (nA-fA) creates a voltage at the feedback resistor which shows up at the output-In values: Ib- = 10nA, therefore 0.1 mV across R2, with Eout = 10V that means an error of 0.001%; therefore the error is rather small in this case

Non-inverting amplifier or follower

Unity gain non-inverting amp is used as a Buffer And for impedance matching between a high source impedance and a low-impedance input circuit

Non-inverting amplifier or follower

Input - Output characteristic of a non-inverting amplifier

Non-inverting amplifier

Non-inverting amplifierand errors

Details in circuit displayed in fig 7-8 Input signal drives very high internal impedance (Rcm, Rdiff etc.).Therefore

very little gain error is induced Small gain error is produced by the voltage divider consisting of Ro and RL Furthermore additional gain errors are created through the bias currents

flowing through the feedback resistances (Ib- and Ib+)

Bias currents correlate to ambient temperature Fig 7-10 provides an overview

concerning the influence fromambient temperature to bias current

Non-inverting amplifier Example

ph probe amplifier

Summing amplifier

Summing amplifier

It is used to remove undesirable dc voltage from a signal.

Vo=0 if=0 ij+ib=0

Differential amplifier

Produces an output voltage proportional to the difference between the voltage applied to the two input terminals

The voltage gain is the same as for inverting followers when the ratio of feedback resistor to input resistor is equal at both terminals.

Unity gain when all four resistor are equal Removes common-mode noise and amplifying the differential signal.

One op-amp differential amplifier

U4

U3

Differential amplifier

The input resistance of one op amp differential amplifier is to low for high-resistance source. Satisfactory for low-resistance source such as Wheatstone bridge

Solution: add two non-inverting gain followers of high input resistance Instrumentation amp has also higher gain

Differential Gain of the two non-inverting combined followers:

One op-amp differential amplifier

Three op-amp differential amp or Instrumentation amplifier

Instrumentation Amplifier

Sensors and Op-amp Examples

Transimpedance amplifier

current to voltage converter A positive input current pulse produces a negative output voltage The If is almost equal to Iin since Ib is small Example (fig): 10nA input gives 0.1V output Most common bioelectric amp is the photodiode amplifier

Integrator - a low pass filter

Gives as an output the integral of an input When a voltage is applied to the integrator, a current I2 begins to charge

C1.

It is function as a low-pass filter with frequency response: The gain decreases as f (f=2f) increases

Differentiator - a high pass filter

Gives as an output the differential of an input

It is function as a high-pass filter with frequency response: The gain increases as f (f=2f) increases

Input Output

Active filters

Frequency Response:

Comparators

Compares the input voltage with some reference voltage and gives in the output positive or negative saturation limits of the op-amp

Comparators

Schmitt Trigger Comparator

Operational amplifiersOperational amplifiersOperational amplifiersOperational amplifiersBasic amplifier configurationsInverting amplifier or followerInverting amplifier or followerInverting amplifierError sources - Inverting amplifierError sources - Inverting amplifierNon-inverting amplifier or followerNon-inverting amplifier or followerNon-inverting amplifierNon-inverting amplifierand errorsNon-inverting amplifier ExampleSumming amplifierSumming amplifierDifferential amplifierDifferential amplifierInstrumentation AmplifierSensors and Op-amp ExamplesTransimpedance amplifierIntegrator - a low pass filterDifferentiator - a high pass filterActive filtersComparatorsComparatorsSchmitt Trigger Comparator