Operational amplifiers

Transcription

1 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 highamplitude bioelectric events Medium-gain amplifiers (x10 to x1000) Recording of ECG waveforms, muscle potentials etc. High-gain amplifiers (x1000 up to x10 6 ) Sensitive measurements, like recording EEG (brain potentials)

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

3 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

4 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)

5 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)

6 Inverting amplifier or follower

7 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)

8 Inverting amplifier

9 Error sources - Inverting amplifier Fig. 7-4 shows detailled circuit of an inverting amplifier Bias currents I b- and I b+ and output load current I o Three types of internal resistance and capacitance (1) Common-mode R cm and C cm, referring to internal ground V ee (2) Differential R diff and C diff between positive and negative input (3) output R o Internal ground reference V ee as middle of positive and negative supply Errors through external components R s 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 R o creates another gain error through voltage divider behavior with the load resistance of the following stage - In this case R l is large enough, so the influence from R o isn t strong enough

10 Error sources - Inverting amplifier Errors through internal components Rcm (is parallel with R1) causes small errors, as it is usually > 1000MΩ Through C cm (< 5pF) higher gain errors will be produced in higher frequencies (Rc=1/jωc) -Example: at 1 Mhz C cm 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

11 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

12 Non-inverting amplifier or follower Input - Output characteristic of a non-inverting amplifier

13 Non-inverting amplifier

14 Non-inverting amplifier and errors Details in circuit displayed in fig 7-8 Input signal drives very high internal impedance (R cm, R diff etc.).therefore very little gain error is induced Small gain error is produced by the voltage divider consisting of R o and R L Furthermore additional gain errors are created through the bias currents flowing through the feedback resistances (I b- and I b+ ) Bias currents correlate to ambient temperature Fig 7-10 provides an overview concerning the influence from ambient temperature to bias current

15 Non-inverting amplifier Example ph probe amplifier

16 Summing amplifier

17 Summing amplifier It is used to remove undesirable dc voltage from a signal. Vo=0 if=0 ij+ib=0

18 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. U3 U4 One op-amp differential amplifier

19 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

20 Instrumentation Amplifier

21 Sensors and Op-amp Examples

22 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

23 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

24 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

25 Active filters Frequency Response:

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

27 Comparators

28 Schmitt Trigger Comparator