IFB270 Advanced Electronic Circuits

Transcription

1 IFB270 Advanced Electronic Circuits Chapter 14: Special-purpose op-amp circuits Prof. Manar Mohaisen Department of EEC Engineering

2 eview of the Precedent Lecture Introduce the level detection op-amp circuits Introduce the op-amp comparators and their applications Introduce the summing op-amp amplifiers and some applications Introduce the integrator and differentiator op-amp circuits Keywords 2

3 Lecture Objectives Introduce the instrumentation amplifier +Applications Introduce the isolation amplifiers +Applications Introduce the operational transconductance amplifier +Applications Introduce the op-amp converters and other circuits Keywords 3

4 Instrumentation Amplifiers Instrumentation amplifier It amplifies the difference between its two inputs The main objective is to amplify a small signal that might be riding on a large common-mode voltages Characteristics of the instrumentation amplifier High input Z, low output Z, high common-mode mode rejection, low output offset 4

5 Instrumentation Amplifiers contd. Instrumentation amplifier A1 and A2 op-amps are non-inverting amplifiers with both high input impedance and gain A3 is a unity gain differential amplifier with equal resistors ( 3 = 4 = 5 = 6 ) G is set to control the closed-loop voltage gain 1 = 2 = A cl = 1+ 2 G G = 2 A 1 cl 5

6 Instrumentation Amplifiers contd. Instrumentation amplifier in1 cm Derivation of the formula of the voltage gain Stage I V + V Vout2 ( Vin2 + Vcm) Vin 1+ Vcm ( Vin2 + Vcm) + = 0 Vin2 + Vcm 2 G V out2 = 1 + V in2 V in1 + V cm 6 1 G V = V = V G + 2 Same way, we find that Stage II Overall gain 1 cm G G V 1 out1 = + V in1 V in2 + V V out V x V Vx + out1 = 0 V out = 2V x Vout 1 = Vout2 Vout out = + in2 in1 G V 1 ( V V ) x out2 out

7 Applications Instrumentation Amplifiers contd. These amplifiers are generally used to measure the difference between two small signals These small signals are superimposed on the common-mode signal Application include situations where quantities are measured by a remote device Measure temperature, pressure, etc. 7

8 Instrumentation Amplifiers contd. A specific instrumentation amplifier The AD622 instrumentation amplifier Note that gain is inversely prop. to the BW. elation between gain and G G = 50.5 kω 1 A v = k Ω 8

9 Isolation Amplifiers A basic capacitor-coupled isolation amplifier A device that consists of two electrically isolated stages; input and output stages The two stages areisolatedbyanisolation an isolation barrier so that a signal must be processed at the first stage before being coupled to the second stage Isolation amplifiers use optical coupling (electronics I), transformer coupling, or capacitive coupling The two stages use different voltage sources and grounds 9

10 Isolation Amplifiers contd. A basic capacitor-coupled isolation amplifier Modulation: An information signal modulates (changes) a characteristic of a carrier signal Characteristic: Amplitude, phase, frequency The isolation barrier is a small-value capacitor Demodulation: ecovers the original modulating signal 10

11 Isolation Amplifiers contd. A basic capacitor-coupled isolation amplifier Amplitude or pulse-width modulation is possible (b) represents an isolation amplifier with a pulse-width modulator Example: IS0124 with a unity gain 11

12 Isolation Amplifiers contd. A transformer-coupled isolation amplifier Texas Instruments (Burr-Brown) 3656KG The gains of both stages are adjusted externally using the resistors AD208 The gain of the input stage can only be controlled It requires an external square wave to drive an output stage power converter f 1 Av 1 = + i1 1 f 2 v2 1 + i2 3656KG A 2 = + A = A A vtot ( ) v 1 v 2 12

13 Applications In medical applications Isolation Amplifiers contd. Heart rate and blood pressure signals are monitored in presence of high common- mode signals In these applications, without isolation, dc leakage or equipment failure could be fatal 13

14 Operational Transconductance Amplifiers (OTA) Conventional op-amp A voltage amplifier where the output voltage is a scaled version of the input voltage OTA A voltage-to-current amplifier where the output current equals the input voltage times the gain The double circle represents a current source that t is dependent d on a bias current OTA has a high input impedance, high CM, bias-current input terminal, a high output impedance, and no fixed open-loop voltage gain 14

15 Operational Transconductance Amplifiers contd. Transconductance Transconductance of an electronic device is the ratio between the output current to the input voltage Transconductance is the gain of the OTA The voltage-to-current to gain is given by g m = I V out in The transconductance is a function of the bias-current gm = KI BIAS Therefore, I = g V = KI V BIAS out m in BIAS in 15

16 Operational Transconductance Amplifiers contd. Basic OTA circuits The transconductance is defined by the amount of bias-current The bias-current depends on the bias voltage and bias resistor gm = KI Therefore, the voltage gain can be controlled by the amount of bias-current Vout Iout L V I A = = = g = out out v Vin Vin L m L BIAS 16

17 Operational Transconductance Amplifiers contd. A specific OTA LM13700 is a representative device of the OTAs Bias-current I BIAS V = BIAS ( V ) 14V 1.4V BIAS The 1.4 V is due to the internal circuit A base-emitter emitter junction and a diode connect BIAS to the negative supply voltage 17

18 Operational Transconductance Amplifiers contd. A specific OTA Example 14-6: Find the gain V BIAS = +V = 9V K = 16 μs/μa Solution: I BIAS V ( V) 1.4V 503 μ A = BIAS = BIAS g m = KI = mS BIAS A = g = (8.05)(10) = 80.5 v m L 18

19 Operational Transconductance Amplifiers Applications First application: Amplitude modulator The input is the fixed-amplitude carrier (sinusoidal of fixed frequency) The bias input is connected to the modulating signal (information signal) The amplitude of the output voltage is a function of the bias voltage I BIAS V ( V) = MOD 1.4V BIAS 19

20 Operational Transconductance Amplifiers Applications contd. First application: Amplitude modulator contd. Example 14-7: Determine the output signal Input: 1MHz with peak-to-peak voltage 50 mv I V BIAS(max) VMOD(max) ( V) 1.4 VMOD(min) ( V) 1.4 = = 314μA IBIAS(min) = = 154μA BIAS BIAS g 246mS m = KIBIAS(max) = 5.02 ms gm = KIBIAS(min) = 2.46 Av (max) = gm L = 50.2 Av (min) = gm L = 24.6 = A V = 2.51 V Vout(min) = Av Vin = 1.23 V peak to peak out(max) peak to peak v in peak to peak peak to peak 20

21 Operational Transconductance Amplifiers Applications contd. Second application: Schmitt Trigger It is a comparator with hysteresis The input is large enough to drive the device into saturation When the input is larger than one threshold value, the device switches to one of its saturation outputs When the input is below another threshold value, the device switches to the second saturation output 21

22 Operational Transconductance Amplifiers Applications contd. Second application: Schmitt Trigger The maximum output current equals the bias-current (I BIAS ) The threshold is set by the current through 1 The upper trigger point is +I BIAS 1 The lower trigger point is I BIAS 1 22

23 Converters and Other Op-Amp Circuits Constant-current source The current in the load is defined by the input circuit components; V IN and i Independent of the value of the load resistor V I = L IN i 23

24 Converters and Other Op-Amp Circuits contd. Current-to-voltage converter An application is depicted in (b) V = I out i f The current through f is controlled by the amount of light a photoconductive device is exposed to. 24

25 Converters and Other Op-Amp Circuits contd. Voltage-to-current converter The input voltage = the voltage drop at the resistor 1 The load current is equal to the current through 1 (input current = 0) I L = V in 1 25

26 Converters and Other Op-Amp Circuits contd. Peak detector The capacitor charges to a value = to the maximum input voltage When the input is less than that charged value, the diode is reverse-biased The peak value is still saved in the capacitor and can be measured Application Measure the maximum value of a surge voltage 26

27 Keywords and terms Instrumentation op-amp amplifier Applications Isolation op-amp amplifier Applications Keywords Operational transconductance amplifier Applications Op-amp constant-current source Op-amp current-to-voltage to converter Op-amp voltage-to-current converter Op-amp peak detector 27

28 Lecture Summary Introduced the instrumentation amplifier Introduced the isolation amplifiers Introduced the operational transconductance amplifier Introduced the op-amp converters and other circuits 28