Introduction to the TL07x Family of Op Amps

The TL071, TL072, and TL074 are popular JFET-input operational amplifiers from Texas Instruments. They offer excellent performance characteristics including:

• High Input Impedance (1012 Ω)
• Low input bias current (65 pA max)
• Wide bandwidth (3 MHz)
• High slew rate (13 V/μs)
• Low noise (18 nV/√Hz)

The different models vary in the number of op amps per package:

These op amps are well-suited for a wide range of analog applications such as active filters, amplifiers, integrators, comparators, and more. Let’s take a closer look at some common circuits built with the TL07x series.

Non-Inverting Amplifier Circuit

One of the most basic op amp circuits is the non-inverting amplifier. It provides voltage gain without inverting the polarity of the input signal. Here is the schematic for a non-inverting amplifier using a TL071:

[Schematic]

The voltage gain (Av) is set by the ratio of the feedback resistor (Rf) to the input resistor (Rin):

Av = 1 + (Rf / Rin)

For example, if Rf = 10 kΩ and Rin = 1 kΩ, the gain would be:

Av = 1 + (10k / 1k) = 11 V/V

A few key points about this circuit:

• The non-inverting input (+) connects to the input voltage source (Vin)
• The inverting input (-) connects to a Voltage Divider formed by Rf and Rin
• The output voltage (Vout) is in-phase with the input but amplified
• Gain is easily set by choosing appropriate Resistor Values
• Input impedance is equal to the impedance looking into the non-inverting input (very high)

Inverting Amplifier Circuit

The inverting amplifier is another fundamental op amp circuit. As the name suggests, it inverts the polarity of the input signal in addition to providing voltage gain. A typical inverting amplifier using a TL072 looks like this:

[Schematic]

In this configuration, the voltage gain is determined by the negative ratio of the feedback resistor to the input resistor:

Av = – (Rf / Rin)

So if Rf = 100 kΩ and Rin = 10 kΩ, the resulting gain would be:

Av = – (100k / 10k) = -10 V/V

Some important characteristics of inverting amplifiers:

• The non-inverting input connects to ground
• Input voltage source connects to Rin, which then connects to the inverting input
• Output voltage is inverted and amplified compared to input
• Gain is negative and set by resistor ratio
• Input impedance is equal to value of Rin

Active Low-Pass Filter

Active filters are an important application of op amps. By combining the high input impedance and gain of an op amp with frequency-selective passives like capacitors, op amps enable the creation of high-performance filter circuits.

A basic active low-pass filter can be built with a single TL074 stage as follows:

[Schematic]

This is a second-order Sallen-Key low-pass topology. The cutoff frequency (Fc) is set by the resistor and capacitor values:

Fc = 1 / (2π√(R1•R2•C1•C2))

And the Q factor depends on the ratio of resistors:

Q = √(R2/R1)

Typical values might be R1 = R2 = 10 kΩ and C1 = C2 = 1 nF, resulting in:

Fc = 1 / (2π√(104 • 104 • 10-9 • 10-9)) = 15.9 kHz
Q = √(104/104) = 1

A Q of 1 provides a Butterworth frequency response. Other Q values create different responses like Bessel or Chebyshev.

Filter design can be complex, but there are many online tools and calculators to help choose component values for desired filter characteristics. Active filters have several benefits over passive types:

• Gain and filtering in one stage
• Easy cascading of multiple stages
• High input and low output impedance
• No loading between stages
• Op amp gain compensates for insertion loss

Integrator and Differentiator Circuits

Op amps can also be used to perform mathematical operations on analog voltages, such as integration and differentiation. An op amp integrator has a capacitor in the feedback path:

[Schematic]

The output voltage is proportional to the integral of the input voltage:

Vout = -(1/RC)∫Vin dt

For a constant input voltage, the output is a ramp with a slope of Vin/RC. Integrators are used in analog computer, control system, and signal processing circuits.

An op amp differentiator uses a capacitor at the input and a resistor in the feedback path, like this:

[Schematic]

Here the output voltage is proportional to the derivative or rate of change of the input:

Vout = -RC dVin/dt

Differentiators find use in applications needing to sense change or motion. However, they tend to amplify high frequency noise, so their bandwidths are usually limited. In general, differentiators are more difficult to apply than integrators.

Voltage Comparator

By operating without negative feedback, an op amp can be used as a comparator to sense when an input voltage crosses a threshold. A basic comparator circuit looks like this:

[Schematic]

The two inputs are:

• Vsig = signal voltage to be compared
• Vref = reference threshold voltage

The output (Vo) will swing to the positive or negative Power Supply rail depending on the relative values of the two inputs:

If Vsig > Vref, then Vo = +Vcc
If Vsig < Vref, then Vo = -Vee

Comparators are used in many applications as analog-to-digital interfaces including:

• Over/undervoltage detection
• Window comparators
• Zero-Crossing Detectors
• Analog-to-digital converters (ADCs)
• Pulse-width modulators (PWM)

Comparators usually have an open-drain output with a pullup resistor so they can drive logic circuits. The TL07x op amps, with their high speed and low input offset voltage, work well as comparators.

Astable Multivibrator

The versatile op amp can also be used to create oscillator circuits that generate periodic analog waveforms. One example is the astable multivibrator, which uses positive feedback and an RC network to produce a square wave output:

[Schematic]

It works like this:

1. C charges through R1 and R2
2. When voltage on C reaches upper threshold (set by R1/R2 ratio), the op amp output goes high
3. This turns on the discharge transistor Q1, rapidly discharging C
4. When voltage on C drops to the lower threshold, the op amp output goes low
5. Q1 turns off and C begins charging again
6. Process repeats indefinitely

The charge/discharge cycle of the capacitor generates a square wave at Vo. The frequency is set by the R1, R2, and C values:

f = 1 / (1.38 • C • (R1+2R2))

And the duty cycle is determined by the ratio of R1 to R2:

Duty Cycle = R2 / (R1 + 2R2)

The TL07x op amps are a good choice for oscillators and multivibrators because of their wide bandwidth, high slew rate, and short settling times.

Window Comparator

For applications that need to check if a signal lies within a certain voltage range, a window comparator is used. It requires two op amp comparator stages:

[Schematic]

The lower op amp compares the input signal to the lower threshold voltage (Vref_low). Its output will be high when Vsig > Vref_low.

The upper op amp compares the input to the upper threshold (Vref_high). It outputs high when Vsig < Vref_high.

The AND gate combines the outputs of the two comparators. Its output will only be high when Vsig is between the two thresholds:

Vref_low < Vsig < Vref_high

A window comparator is useful for detecting when a signal is within acceptable limits. It can monitor parameters like:

• Power supply voltages
• Battery charge level
• Temperature
• Pressure
• Position

Using a quad op amp package like the TL074 allows the whole window comparator circuit to be built with a single chip.

Instrumentation Amplifier

When small signals from sensors need to be amplified in the presence of noise and interference, an instrumentation amplifier offers superior performance compared to basic op amp circuits. A 3 op amp instrumentation amplifier can be built with the TL074 like this:

[Schematic]

The key features are:

• High input impedance due to non-inverting buffers on each input
• Adjustable gain set by R_gain
• High CMRR (common-mode rejection ratio) to reject interference
• Low DC offset and drift
• Single resistor gain adjustment

The transfer function is:

Vout = (1 + 2R1/Rgain) • (V2 – V1)

Where V1 and V2 are the voltages on the two input terminals. The gain is set by the value of Rgain:

Gain = (1 + 2R1/Rgain)

Typical values are R1 = 10 kΩ and Rgain = 100 Ω to 10 kΩ, providing gains from 1000 down to 10.

Instrumentation amplifiers are widely used as interfaces for:

• Strain gauges
• Thermocouples
• Resistance temperature detectors (RTDs)
• Load cells
• Electrocardiogram (ECG) electrodes
• Other low-level sensors and transducers

The high performance of the TL07x series makes them an excellent choice for building instrumentation amplifiers.

Frequently Asked Questions

What is the main difference between the TL071, TL072, and TL074?

The primary difference is the number of op amp circuits per package. The TL071 is a single op amp, the TL072 is a dual op amp, and the TL074 contains four op amps. Electrically, they have nearly identical specifications.

Can the TL07x op amps be used with single supply voltages?

Yes, the TL07x series can operate from single supplies as low as 5 V. However, the inputs and outputs are not rail-to-rail, so they cannot swing all the way to ground or the positive supply. Special techniques like biasing the non-inverting input to mid-supply are needed for single-supply operation.

What is the maximum supply voltage for the TL07x?

The absolute maximum supply voltage is ±18 V. The recommended operating range is ±5 V to ±15 V. Running above ±15 V will cause increased power dissipation and may overheat the part.

How do the TL07x op amps compare to newer rail-to-rail types?

The TL07x are JFET-input op amps, while many newer types use CMOS input stages. The JFET design gives the TL07x their very high input impedance, low bias currents, and low noise. Newer CMOS op amps have lower supply current and support rail-to-rail operation, but usually have higher noise and lower impedance. The TL07x still offer excellent performance, especially for low-frequency applications.

Can the TL07x be directly replaced by other op amps?

In many cases, yes. The TL07x pin-out and basic specifications are similar to many other op amps like the LF347 or OP-470. However, always check the datasheet for exact specifications before substituting another op amp. Some parameters like input bias current, offset voltage, noise, and supply range may be different and could affect circuit operation.

Conclusion

The TL071, TL072, and TL074 are classic JFET op amps that have been popular choices for over 40 years. Their excellent combination of high input impedance, low noise, and good speed make them suitable for a wide range of applications in analog signal conditioning and processing. While newer op amp technologies have some advantages, the TL07x series continues to be widely used and is still an excellent choice for many designs.