# Capacitance Bridge

This circuit is a capacitance bridge; it detects the matching between a reference capacitance C1 and an unknown capacitance, Cx. It uses an instrumentation amplifier in an unusual topology– a sine wave drives the two input op amps’ non- inverting inputs (ordinarily the IA inputs) and the internal 25k feedback resistor of each op amp forces that sine wave to appear at the op amp’s inverting inputs (ordinarily the IA gain resistor connections). The impressed AC voltage across each capacitor causes current to flow in each feedback resistor and this creates a voltage at the output of each of the two input op amps. The third IA stage, a differential amplifier, subtracts the two voltages. Thus, when Cx = C1, the output voltage is zero.  A higher or lower capacitance at Cx unbalances the bridge and an output results that is proportional to the capacitance difference; a high/low mismatch is indicated by a 180 degree phase difference. A synchronous detector (aka phase- sensitive demodulator) driven by F and low- pass filtered will result in a capacitance bridge DC output that is at null when Cx = C1.  The sensitivity of the bridge is proportional to F, both in amplitude and frequency, and to the reference capacitance C1. Higher frequencies result in higher output voltages but the inevitable IA CMRR roll- off at high frequencies will reduce the depth of the null voltage.  An advantage of this circuit is that it is quite simple and it allows both capacitors to be ground- referenced.  (Circuit is created by Neil P. Albaugh,  TI-Tucson)

Capacitance Bridge circuit:

## Online Simulation of the “Capacitance Bridge” Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

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Michael Koltai
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# Triangle & Square Wave Oscillator 1kHz

This simple relaxation oscillator provides both a square- wave and a triangular- wave output. This oscillator is biased for operation on a single +5V supply. The DC component of each output can be removed by capacitive coupling if necessary.Since a rail-to-rail output op amp is used for U1, the square wave output amplitude is the same as its supply voltage. Alower quiescent current op amp such as an OPA364 can be used if Iq is important. Oscillator frequency is determined by C1 & R1. The Transient Analysis used the “Zero initial conditions” to aid the oscillator start- up. This start- up time is visible for the first few milliseconds in the waveform above. (Circuit is created by Neil P. Albaugh  TI- Tucson)

“Triangle & Square Wave Oscillator 1kHz” circuit:

## Online Simulation of the “Triangle & Square Wave Oscillator 1kHz” Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
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# Programmable-Brightness LED Control

This simple op amp circuit can be used to control the brightness of a LED. By placing the LED in the op amp feedback, it is driven  in a constant- current mode. This eliminates the diode forward voltage temperature coefficient’s effect on its current and thus its  brightness over temperature.  This circuit is a voltage- controlled current source. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Programmable-Brightness LED Control circuit:

## Online Simulation of the Programmable-Brightness LED Control Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
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# Instrumentation Amplifier Offset Correction Loop

The feedback from integrator U2 provides a DC offset nulling function to the instrumentation amplifier (IA) U1. Although the IA response is similar to an AC- coupled amplifier, its input is, in fact, still DC- coupled and its input common-mode voltage limits must be observed.

Dc response can be preserved if a switch is added in series with R1. With the switch momentarily closed, the loop error is nulled and stored on C1 when the switch is open.
The switch converts the integrator into a sample/hold amplifier. To minimize correction voltage droop due to bias current, a JFET op amp such as an OPA132 is recommended for S/H use. Bypass capacitors are not shown. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Instrumentation Amplifier Offset Correction Loop circuit:

## Online Simulation of the Instrumentation Amplifier Offset Correction Loop Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

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You can send this link to any TINACloud customers and they can immediately load it by a single click and then run using TINACloud.

Michael Koltai
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# Fast Single-Supply Peak Detector

Notes:

1. Frequency compensation is determined by C1, C2, R2, and the sum of R1 and the forward resistance of D1. Since the dynamic resistance of D1 varies with current the peak detector must be analyzed for stability over its full output amplitude range.

2. The droop rate of the peak detector is determined by the input bias current of U1 plus the input bias current of the output buffer amplifier (not shown).

3. The input voltage range of 0 to +3.5V is limited by the CMV range of U1. R3 protects the op amp input from damage when the input voltage swings negative. (Circuit is created by Neil P. Albaugh  TI – Tucson)

## Online Simulation of the Fast Single-Supply Peak Detector Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediately load it by a single click and then run using TINACloud.

Michael Koltai
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# Single-Supply Bipolar-Input Differential Output Amplifier

The rail- to- rail input and output characteristics of these CMOS op amps allow them to swing very close to their supply rails– +5V and ground.
By using both an inverting and noninverting amplifier output to swing only positive due to their not being capable of swinging below ground (0V), the op amps each act like a perfect rectifier. Due to its unique R-R input topology, the OPA364 exhibits very high linearity over its entire common- mode input voltage range. This absolute- value amplifier has a gain of 1V/V and has an input range of within a few mV of -5V to +5V. (Circuit is created by Neil P. Albaugh,  TI- Tucson)

## Online Simulation of the Single-Supply Bipolar-Input Differential Output Amplifier Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud  and analyze the circuit, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediately load it by a single click and then run using TINACloud.

Michael Koltai
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This circuit is a voltage- controlled current sink. It is scaled to provide a 500mA output current with a +1V input voltage. This type of current sink can be very useful in power supply testing applications. A R-R output op amp with an input common-mode range that includes its negative supply rail, such as an OPA251, is required for single- supply operation. Re- scaling this circuit with other Darlington transistors or low- threshold N-channel MOSFETs can result in an output current sink capability of many amps. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

## Online Simulation of the Voltage-Controlled Electronic Load Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

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You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
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# Differential Amplifier Resistor Tolerance Analysis

Make all resistors “Control Objects” and use “Parameter Stepping” to step each resistor value from 9.9k (1% low) to 10.1k (1% high) in 3 linear steps. Run DC Analysis, “DC Transfer Characteristic” and sweep “Vcmv” from -1V to +1V. The resulting family of curves shows the differential amplifier output error due to the various resistor tolerance combinations. The OPA277 error contribution is nil. Note that using 1% resistors in a differential amplifier design can result in a worst- case CMRR error of 20mV per volt of common-mode voltage. This is only 36dB! (Circuit is created Neil P. Albaugh  TI-Tucson)

## Online Simulation of the Differential Amplifier Resistor Tolerance Analysis Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

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Michael Koltai
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# Positive Output Precision Voltage Limiter

This circuit limits its output to a positive- going output only; negative output is clamped to ground. For negative inputs, D1 conducts and R2 provides negative feedback into U1’s summing junction. For positive inputs, D2 conducts and holds the summing junction to 0V. Thus the output across RL can only be positive. This characteristic is handy when driving single-supply amplifiers or unipolar A/D converters. For a negative output simply reverse D1 & D2. As shown, this circuit is a unity- gain inverter but it is also capable of providing voltage gain. Av = – 2 / R1. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Positive Output Precision Voltage Limiter circuit:

## Online Simulation of the Positive Output Precision Voltage Limiter Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud  and analyze the circuit yourself, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
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# Online Simulation a Transimpedance Amplifier Circuit

This fast photodiode transimpedance amplifier is based on a high- speed JFET- input op amp OPA657. This op amp is compensated for a minimum closed- loop gain of 7V/V but the capacitance of the photodiode plus the op amp input capacitance together with the feedback resistor R1 provides a noise gain at high frequency that allows stable operation. Compensation capacitor C1 optimizes the amplifier bandwidth / gain peaking tradeoff. Achieving this level of performance requires very careful layout and the circuit must be shielded to prevent noise pickup. (Circuit is created by Neil P. Albaugh,  TI- Tucson)

Transimpedance amplifier circuit:

## Online Simulation a Transimpedance Amplifier Circuit

The great feature of the TINA circuit simulator that you can analyze this circuit immediately with TINACloud the online version of TINA. Of course you can also run this circuit in the off-line version of TINA.

Click here to invoke TINACloud and analyze the circuit yourself, or watch our tutorial video!

You can send this link to any TINACloud customers and they can immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
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