Circuit diagram of wideband ALC amplifier with TL026 and other components

Building a differential amplifier

An op-amp and no feedback is already a differential amplifier, amplifying the voltage difference between the inputs. However, its gain cannot be controlled, and it's generally too high to be of any practical use. So far, we application of negative feedback to op-amps have resulting in the practical loss of one of the inputs, the ResU Lting amplifier only good for amplifying a, single voltage signal input. With a little ingenuity, however, we can construct a op-amp circuit maintaining both voltage inputs, yet with a Controlle D gain set by external resistors.

If all the resistor values is equal, this amplifier would have a differential voltage gain of 1. The. Circuit is essentially the same as a inverting amplifier, except that the noninverting input (+) of the op-amp is at a voltage equal to a fraction of V2, rather than being connected directly to ground. As would stand to reason, V2 functions as the noninverting input and V1 functions as the inverting input of the final Ampl Ifier Circuit. Therefore:

If we wanted to provide a differential gain of anything other than 1, we would has to adjust the resistances in both< /c0> Upper and lower voltage dividers, necessitating multiple resistor changes and balancing between the both dividers for Symmetrical operation. This isn't always practical, for obvious reasons.

Another limitation of this amplifier design was the fact that its input impedances be rather low compared to that of some Other Op-amp configurations, most notably the noninverting (single-ended input) amplifier. Each of the input voltage source has through a resistance, which constitutes far less impedance than the bare in Put of an op-amp alone. The solution to this problem, fortunately, was quite simple. All we need to does is "buffer" for each input voltage signal through a voltage follower like this:

Now the V1 and V2 input lines is connected straight to the inputs of both voltage-follower op-amps, giving very high imped Ance.

The op-amps on the left now handle the driving of the current through the resistors instead of letting the input voltage s Ources

(whatever they may is) do it. The increased complexity to our circuit are minimal for a substantial benefit.

The instrumentation Amplifier

As suggested before, it's beneficial to being able to adjust the gain of the amplifier circuit have to change more than one resistor value, as is necessary with the previous design of differential amplifier. The so-called instrumentationbuilds on the last version of differential amplifier to give us that capability:

This intimidating circuit are constructed from a buffered differential amplifier stage with three new resistors linking the Both buffer circuits together.

Consider all resistors to is of equal value except for Rgain.

The negative feedback of the upper-left op-amp causes the voltage at point 1 (top of Rgain) to being equal to V1.

Likewise, the voltage at point 2 (bottom of Rgain) was held to a value equal to V2.

This establishes a voltage drop across rgain equal to the voltage difference between V1 and V2.

That's voltage drop causes a current through rgain, and since the feedback loops of the the the both input op-amps draw no current,

That's same amount of current through Rgain must is going through the "R" resistors above and below it.

This produces a voltage drop between points 3 and 4 equal to:

The regular differential amplifier on the right-hand side of the circuit then takes this voltage drop between points 3 and 4, and amplifies it by a gain of 1 (assuming again-all "R" resistors is of equal value). Though this looks-like a cumbersome-to-build a differential amplifier, it has the distinct advantages of possessing ex tremely High input impedances in the V1 and V2 inputs (because they connect straight into the noninverting inputs of their respective op-amps), and adjustable gain that can is set by a single resistor. Manipulating the above formula a bit, we have a general expression for overall voltage gain in the instrumentation Amplifi Er

Hough It may is obvious by looking at the schematic, we can change the differential gain of the instrumentation Amplif Ier simply by changing the value of one resistor:rgain. Yes, we could still the overall gain by changing the values of some of the other resistors, but this would necessit Ate balanced resistor value changes for the circuit to remain symmetrical. Please note this lowest gain possible with the above circuit are obtained with Rgain completely open (infinite Resistan CE), and that gain value is 1.

An instrumentation amplifier are a differential op-amp circuit providing high input impedances
With ease of gain adjustment through the variation of a single resistor.

Voltage definitions

To understand the behavior of a fully-differential amplifier, it's important to understand the voltage definitions used T o Describe the amplifier.

Figure 3 shows a block diagram used to represent a fully-differential amplifier and its input and output voltage Definitio Ns.

The voltage difference between the plus and minus inputs is the input differential voltage, Vid.

The average of the voltages is the input common-mode voltage, Vic.

The difference between the voltages at the plus and minus outputs are the output differential voltage, Vod.

The output Common-mode voltage, Voc, is the average of the output voltages, and are controlled by the voltage at VOCM.

With a (f) as the frequency-dependant differential gain of the amplifier and then Vod = Vidxa (f).

Basic circuits

In a fully-differential amplifier, there is, possible feedback paths in the main differential amplifier Side.

This naturally forms inverting amplifiers, and inverting topologies is easily adapted to fully-differential amplifier S.

Figure 6 shows-Configure a fully-differential amplifier with negative feedback to control the gain and maintain a B alanced amplifier.

Symmetry in the and feedback paths is important to has good CMRR performance.

CMRR is directly proportional to the resistor matching error-a 0.1% error results in.

The VOCM error amplifier is independent of the main differential amplifier.

The action of the VOCM error amplifier is to maintain the output common-mode voltage at the same level as the voltage Inpu T to the VOCM pin.

With symmetrical feedback, output balance are maintained, and vout+ and vout–swing symmetrically around the voltage at the VOCM input.

Generation of differential signals have been cumbersome in the past.

Different means has been used, requiring multiple amplifiers.

The integrated fully-differential amplifier provides a more elegant solution.

Figure 7 shows a example of converting single-ended signals to differential signals.

A Simple IF AGC circuit This features wide dynamic range and excellent linearity can be achieved with chips:

Tl ' s tl026c voltage-controlled amplifier IC and Linear technology ' s LT1014 (or any other similar basic quad op amp).

Features of wideband ALC amplifier circuits with 50mhz/-3db bandwidth and 20DB compression characteristics

This is a circuit that stabilizes input level signals to a certain level, and is used in circuits with high performance requirements, where the output level fluctuates due to the uneven frequency characteristics of these signal generators.

If this circuit is added, automatic control can be carried out so that the signal remains certain amplitude. In addition, in order to reduce the output impedance, the circuit added push-pull level.

How the Circuit works

This circuit uses a wideband amplifier IC that can be amplified by an external voltage control, thus having a compression characteristic of 20DB. In the input circuit, a resistor with a ★ Mark is added to reduce the input level,

When driving 50 euro load, because the TL026 difficult to obtain a large output amplitude, so in the circuit is added by the transistor composed of a push-pull buffer amplifier to reduce the burden of TL026.

The output of the TL026 is differential, and if the load resistance is not equal, the frequency characteristics change, so the C2 and R4 are connected to the leads.

The potential difference between the lead 2.7 allows the magnification to be controlled because of the DC drift, so the op amp is used.

The diode D1 the output and compares it to the reference voltage. The diode D2 is added to compensate for the temperature characteristics of the D1.

OP amplifier A2 acts as a comparison circuit, when the output level rises, the current flowing through the D1 will increase, A2 the integral after the output negative voltage,

And added to the A3 of the inverting input, so that the A2 lead 2 in relation to the A1 of the lead 7 of the potential has increased, so that the A1 magnification decreased.

Selection of components

Because the entire circuit form ALC loop, so the selection of components is relatively easy, but the reference voltage of the diode D5, variable resistance VR1, resistance R12, R13 stability is the choice of components should be a key consideration.

In order to make the forward voltage of diode D1 and D2 equal, thermal coupling should be used. Ordinary small signal switching diode, 50MHZ when its rectification characteristics will be reduced, so, should choose Schottky Diode.

Adjustment and electrical characteristics

The resistor with ★ Mark is not added, input -20dbm,f=1mhz signal, adjust VR1, get 1vp-p voltage at the output end.

The input voltage is amplified by 10DB, and the output is verified to be unchanged.

Can I use TL026 as an input amplification stage for a 10-bit ADC and use PWM with RC filter to generate the gain control s Ignal? Thanks.

The PWM is generated based on ADC output.

Hello Frank,

The AGC can run from the filtered PWM signal. Keep in mind the limited AGC range, vref-180mv < VAGC < vref+180mv.

This was shown in both Figure 5 on page 4 and the ' Gain characteristics ' on page 5.

Regards, Ron Michallick

Gain characteristics

Figure 5 shows the differential voltage amplification versus the differential Gain-control voltage (VAGC–VREF).

VAGC is the absolute voltage applied to the AGC input and Vref are the DC voltage at the REF out output.

As VAGC increases with respect to Vref, the tl026c gain changes from maximum to minimum.

As shown in Figure 5 for example, VAGC would has to vary

From approximately-in-a- than Vref to approximately- mv greater than Vref to change the gain FR Om Maximum to minimum.

The total signal change in VAGC are defined by the following equation.

? VAGC = (Vref + +) – (vref–180 mv)

? VAGC = MV (1)

However, because VAGC varies as the AC AGC signal varies and also differentially around Vref,

Then VAGC should has an AC signal component and a DC component.

To preserve the DC and thermal tracking of the device, this DC voltage must is generated from Vref.

To apply proper bias to the AGC input, the external circuit used to generate VAGC must combine these, voltages.

Figures 6 and 7 show circuits that would perform this operation and is easy to implement.

The circuits use a standard dual operational amplifier for AGC feedback.

By providing rectification and the required feedback gain, these circuits is also complete AGC systems.

tl026 noise when input aty GND

My customer uses the tl026c with differencial output and input to GND with +/-6v.

The output is connected to serial caps of 150nF and with 2 K load.

The output is pretty noisy, could you explainit?

I have the sch and plots.

Kamal,

The output is floating.

Try replacing the 2k resistor with a 1k resistors in series then ground the node between the resistors.

Measure noise. Then turn power to TL026 off and measure noise again (power off noise, not caused by TL026).

Regards,

Ronald Michallick
Linear applications

Design of AGC amplifier circuit in video optical receiver using tl026c

TL026C is a differential high-frequency amplifier with automatic gain control (AUTOMATICGAINCONTROL,AGC) capability produced by TI Corporation, USA.

The gain change is controlled by the AGC pin voltage, and the input +200mv voltage to the AGC end relative to the reference voltage (REFOUT) can be obtained with variable gain of 50dB range.

TLD26C is widely used in video and pulse amplification circuits which require wide band, low phase deviation and good gain stability.

The AGC principle of tl026c tl026c internal AGC feedback circuit makes the output signal have wide frequency band, low phase deviation and excellent gain stability.

The change in chip gain varies with the control voltage of the AGC pin, which has a variable gain of 50dB range relative to the reference voltage.

The gain is related to the differential control voltage (V-V) as shown in Figure 1. Wherein the VAGC is the tl026c of the AGC pin voltage,

Vref is the DC voltage of the refout pin output and is a reference voltage with a constant voltage value that does not change with the output voltage of the chip.

The TL026C chip gain is changed when compared to VAGC. As can be seen from Figure 2, the VAGC value from about 180mv to +180MV, the chip gain from the maximum to a minimum.

That is, the chip gain decreases when the relative vs. reference Vref increases, whereas the VAGC gain increases.

In this way, the output signal gain is automatically controlled, and the output signal is kept within a constant range.

AGC Circuit precautions

The conduction voltage of the detector diode determines the threshold detection voltage of the AGC circuit.

The conduction voltage of the silicon tube is about 0. 7V. Therefore, depending on the output, the selected diode must have enough amplitude to pass the output signal.

Due to the limitation of the internal circuit of the TL026C chip, its maximum signal output peak is not more than 3 v.

Circuit diagram of wideband ALC amplifier with TL026 and other components