Operational amplifiers Types of operational amplifiers (bioelectric - - PowerPoint PPT Presentation

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 180º 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 supply

Errors 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 isn’t strong

enough

Error sources - Inverting amplifier

Errors through internal components

• Rcm (is parallel with R1) causes small errors, as it is usually > 1000MΩ
• Through Ccm (< 5pF) higher gain errors will be produced in higher

frequencies (Rc=1/jωc)

• Example: at 1 Mhz Ccm reactance is at 32kΩ, 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 amplifier and 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 from ambient 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=2πf) 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=2πf) 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