The Circuit Design Blog

Bootstrapped Input For High Impedance

Applying a small amount of positive feedback to the input bias current return resistor R3 effectively raises the apparent input resistance seen by an input signal. Without feedback the input resistance is R3 (1M) in parallel with the input resistance of U1 (1E13 ohms); positive feedback applied through the voltage divider R1 & R2 multiplies the effective input impedance of R3 by creating a smaller differential voltage across the resistor. The pole frequencies of various feedback fractions are illustrated by the AC analysis below. A piezoelectric transducer or condenser microphone is modeled by VG1 in series with capacitor C1. Without bootstrapping (positive feedback), the low- frequency cutoff is 1.8kHz but by placing a 100 ohm resistor at R1, this cut-off frequency drops to 2Hz, illustrating the increased Rin. This does not come without penalty, however. Adding bootstrapping also increases the noise gain of the op amp, multiplying its Vos, drift, and noise. Adding a LARGE capacitor in series with R1 can eliminate the amplified DC offset and drift but the low frequency noise will still suffer. Approach large + feedback fractions with caution; instability and susceptibility to external noise pickup can result. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Bootstrapped Input For High Impedance circuit:

Online Simulation of the “Bootstrapped Input For High Impedance” 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
www.tina.com

Voltage-Controlled Current Source Circuit

This circuit is a Howland voltage-controlled current source. It is scaled to provide a 20mA output current with a +1V input voltage. This type of current source can be very useful in industrial 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. For V+ supply over 12V, use Zetex ZXTN2010G (60V, 3W, SOT223, HFE = 100 min). Re- scaling this circuit with other transistors can result in output current capability of a many amps. (Circuit is created by Neil P. Albaugh,  TI – Tucson)

Voltage-Controlled Current Source Circuit:

Online Simulation of the “Voltage-Controlled Current Source” 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
www.tina.com

10Hz Active Low-Pass Filter

This “10Hz Active Low-Pass Filter” circuit offers a very low frequency cutoff and it has the ability to operate on single supply voltages. As shown, this is a second- order unity- gain Butterworth low- pass filter using a Sallen- Key topology. One advantage of a Sallen- Key LPF over one configured as a Multiple Feedback LPF is that it is non-inverting–therefore an input signal with a positive DC offset can be accomodated. A low input bias current op amp is required for a very low frequency low- pass filter as the R values are necessarily high. Thermal noise contributed by these resistors, however, can be reduced by reducing their values by an order of magnitude together with increasing the capacitor values by an order of magnitude. Resistor thermal noise will thereby be decreased by the square- root of 10.  (Circuit is created by Neil P. Albaugh,  TI – Tucson )

10Hz Active Low-Pass Filter circuit:

Online Simulation of the “10Hz Active Low-Pass Filter” 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
www.tina.com

High-Capacitance Line Driver

Capacitive loads invite amplifier instability by reducing phase margin. Instability can be prevented by isolating  the load capacitance from the amplifier output by a small resistor R3. To eliminate a voltage drop error when current is drawn through that resistor, DC feedback is sensed on the load side of R3. High- frequency feedback is provided by C2. The load capacitance was stepped from 100pF to 1uF and the results are shown below; no gain peaking is evident. Bypass capacitors are not shown. (Circuit is created by Neil P. Albaugh  TI-Tucson)

High-Capacitance Line Driver circuit:

Online Simulation of the “High-Capacitance Line Driver” 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
www.tina.com

Simple Absolute Value Amplifier

The rail- to- rail output characteristics of these CMOS op amps allow them to swing very close to their negative supply rails–ground. By using a non-inverting amplifier U1 to swing only positive (due to its not being capable of swinging below ground), this op amp acts like a perfect rectifier. For positive inputs, the input to the inverting amplifier U2 sees a voltage that is equal to the voltage on its non-inverting input (from follower U1), therefore the net gain of U2 is +1V/V. For negative inputs, the + input to the inverting amplifier U2 sees a voltage that is as close as U1 can swing to its negative supply rail (ground); therefore the net gain of U2 is -1V/V. This output of U1 is amplified by the noise gain of U2 and appears as an offset error on the output of the absolute value amplifier.

This is the primary limitation to accuracy with very small input signals.

This absolute- value amplifier has a gain of +1V/V and has an input range of +/- a few mV to -10V to +10V. (Circuit is created by Thomas Kugelstadt & Neil P. Albaugh, TI – Tucson)

Simple Absolute Value Amplifier circuit:

Online Simulation of the “Simple Absolute Value 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 immediatelly load it by a single click and then run using TINACloud.

Michael Koltai
www.tina.com