09/16/2010© 2010 NTUST Today Course overview and information

Instrumentation amplifiers An instrumentation amplifier (IA) is a special integrated circuit designed to for applications where small signals are in noisy environments. They have high input impedance and a high CMRR for excellent noise rejection. R 3 to R 6 are equal values. If R 1 = R 2, then the closed-loop gain is set by a single external resistor, R G, supplied by the user. The gain is: R1R1 V in1 + V cm R2R2 R3R3 R4R4 R5R5 R6R6 +  +  +  V in2 + V cm V out = A cl (V in2  V in1 ) RGRG Instrumentation Amplifier

R1R1 V in1 R2R2 R3R3 R4R4 R5R5 R6R6 +  +  +  V in2 V out RGRG For R 1 = R 2 = 10 k , what value of R G will set the gain to 40? Solving for R G, 513  (510  is the nearest standard value). Instrumentation Amplifier

The bandwidth of instrumentation amplifiers decreases when higher gain is selected k 10k 100k 1M 10M The frequency response of the AD633 is shown. If the highest frequency in the signal is not greater than about 3.5 kHz, the maximum gain of 1000 can be selected. Voltage gain Frequency (Hz) Instrumentation Amplifier

An isolation amplifier provides dc isolation for applications where electrical separation between input and output is necessary. A block diagram of a capacitively coupled isolation amplifier is shown. Notice the separate power supplies and grounds for each stage. +V  V Input stageOutput stage Op-amp ModulatorDemodulator Oscillator Isolation barrier with capacitive coupling Isolation Amplifiers

For the capacitively coupled isolation amplifier, the input signal is modulated. Modulation is the process of modifying the input with another waveform in order to transmit the signal across the barrier. +V  V Input stageOutput stage Op-amp ModulatorDemodulator Oscillator Original input AM PWM Modulated signal Isolation Amplifiers

Transformer coupling can also be used in isolation amplifiers. The Burr-Brown 3656KG is an example of a versatile IC that has gain control as well as three-port isolation (with three separate grounds). It can also supply isolated power on both the input and the output side for external devices. The 3656KG is suited to applications such as interfacing the signals in an electrocardiogram, or to isolate the input signals for fetal heart monitoring as shown here in in the text. Electrode for sensing fetal heartbeat Common electrode Com Shielded cable InputOutput Heart monitor 3656KG Isolation Amplifiers

An operational transconductance amplifier is a voltage to current amplifier. The symbol shows a current source on the output which is dependent on bias current. I BIAS Output Inputs +  The gain of an OTA is given as a transconductance parameter (similar to a FET): The transconductance is dependent on the bias current (I BIAS ) and a constant (K): g m = KI BIAS I out = KI BIAS V in OTA Operational Transconductance Amplifier (OTA)

The specification sheet shows a graph of the relationship between transconductance and bias current. The user can then set the bias to the desired transconductance Bias current (  A) Transconductance, gm (  S) A typical curve is shown. Notice that both axes are logarithmic. For example, a transconductance of 1000  S (10 3 ), requires a bias current of about 63  A. Operational Transconductance Amplifier (OTA)

R1R1 V in R2R2 RLRL +  R BIAS V out 20 k  +15 V  15 V 10 k  The LM13700 is a representative OTA which has a g m that can be set over a six decade range. There are many applications 1 for this OTA including modulators, function generators, and voltage controlled circuits including amplifiers, filters, and resistors. The bias current for the LM13700 is found from the formula 1 see: OTA Operational Transconductance Amplifier (OTA)

What value of bias current will produce a voltage gain of 40 for the OTA? The transconductance curve is shown Bias current (  A) Transconductance, gm (  S) R1R1 V in R2R2 RLRL +  R BIAS V out 20 k  +15 V  15 V The required g m is A v /R L = 40/20 k  = 2,000  S. 10 k  From the graph, I BIAS ≈ 125  A OTA Operational Transconductance Amplifier (OTA)

R1R1 V in R2R2 RLRL +  R BIAS V out 20 k  +15 V  15 V 10 k  What value of bias resistor will set the bias current at 125  A for the previous circuit? = 228 k  The nearest standard 5% value is 220 k . OTA 220 k  Operational Transconductance Amplifier (OTA)

A basic clamping circuit (also called a dc restorer) adds a dc level to a signal voltage using a diode and capacitor. The first negative cycle of the signal biases the diode on and causes the capacitor to charge to V p(in)  0.7 V. The long time constant keeps the capacitor charged, which adds a dc voltage to the signal voltage at the output. V p(in)  0.7 V V in V out + 0 V RLRL  C D Diode conducts Clamping Circuits

Active diode circuits use an op-amp and feedback to closely approximate the behavior of an ideal diode. The same clamping circuit with an active diode has a nearly ideal response. The 0.7 V diode drop in the basic circuit can be a problem for low-level signals, which are common in signal processing applications. RLRL R1R1 C +  V in V out D V in 0 V +  Clamping Circuits

The first positive cycle of the signal charges the capacitor as shown, adding a negative dc voltage to the input. Active clamping circuits can produce negative clamping action by reversing the diode. RLRL R1R1 C +  V in V out D V in 0 V +  Clamping Circuits

Diode limiters (clippers) are circuits that limit voltage above or below a specified level. A basic limiter circuit that clips voltages below 0.7 V is shown. If the input is above 0.7 V, the diode conducts, causing the output to be limited to this level. RLRL R1R1 V out D V in 0 V 0.7 V Limiting Circuits

Reversing the diode causes the limiter to clip signal voltages that are below –0.7 V. If the input is below  0.7 V, the diode conducts, causing the output to be limited (clipped) for voltages less than this level. RLRL R1R1 V out D V in  0.7 V 0 V Limiting Circuits

An active limiter uses an op-amp and diode circuit to form an almost ideal diode. This means the 0.7 V forward drop of the diode does not affect the output. This circuit limits the positive waveform (clipping voltages above ground) because the + input reference is at 0 V. R V out D V in 0 V +  A different reference level is easy to achieve by putting the desired reference voltage on the non-inverting input. Limiting Circuits

Another useful active diode circuit is the peak detector. The purpose of the circuit is to store the maximum positive value of a voltage on a capacitor and hold the value for a certain time. +  R1R1 RiRi C V in D V OUT The op-amp is set up as a comparator. If V in > V C, the diode is forward biased and charges to the peak of V in. For example if a 1.0 V pp sine wave is the input, the output will be a dc level of +0.5 V. Peak Detector

Instrumentation amplifier Isolation amplifier Operational transconductance amplifier (OTA) An amplifier specifically designed for amplifying small differential signals and rejecting large common-mode voltages. A voltage-to-current amplifier in which the gain is set by a bias current. An amplifier with electrically isolated internal stages. Selected Key Terms

Clamper Limiter A circuit that adds a dc level to an ac signal; a dc restorer. A circuit that removes part of a waveform above or below a specified level; a clipper. Selected Key Terms

1. Selecting the highest gain for an instrumentation amplifier means that a. the bandwidth will be less. b. the CMRR is higher. c. both of the above. d. none of the above. Quiz

2. The block diagram for an instrumentation amplifier is shown. The resistor shown in the blue box a.determines the CMRR. b.determines the gain. c.both of the above. d.none of the above. R1R1 R2R2 R3R3 R4R4 R5R5 R6R6 +  +  +  RGRG Quiz

3. For an isolation amplifier, the input and output stages cannot a.have a common power supply. b.be connected with a conductive path. c.both of the above. d.none of the above. Quiz

4. In an isolation amplifier the purpose of the modulator is to a. increase the signal-to-noise ratio. b. increase the bandwidth. c. remove high frequency noise from the signal. d. modify the signal for transmission. Quiz

5. The gain of an operational transconductance amplifier (OTA), is specified as the ratio of a.output current to input current. b.output voltage to input current. c.output voltage to input voltage. d.output current to input voltage. Quiz

6. To increase the gain of an operational transconductance amplifier (OTA), you would a.change the ratio of the feedback resistors. b.reduce the size of the bias resistor. c.increase the size of the gain resistor. d.increase the size of the load resistor. Quiz

7. Another name for a clamping circuit is a a. dc restorer. b. clipping circuit. c. limiter. d. peak detector. Quiz

8. The dc voltage you would expect to measure across the capacitor is equal to 0.7 V less than the a. peak-to-peak value of the input voltage. b. rms value of the input voltage. c. peak value of the input voltage. d. average value of the input voltage. V in V out + RLRL  C D Quiz

9. Reversing the diode in a clipping circuit causes a. the opposite side of the input to be clipped. b. a dc level shift in the output. c. the clipping level to increase. d. the ground reference to change. Quiz

10. Assume the circuit has an ac input as shown. The output will be a. an amplified sine wave. b. 0.5 Vdc c. 1.0 Vdc d. 2.0 Vdc +  R1R1 RiRi C V in D V OUT +1.0 V 0 V  1.0 V Quiz

Answers: 1. a 2. b 3. c 4. d 5. d 6. b 7. a 8. c 9. a 10. c Quiz