採用TL026等構成的寬帶ALC放大器電路圖

Building a Differential Amplifier

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

If all the resistor values are equal, this amplifier will have a differential voltage gain of 1. The analysis of this circuit is essentially the same as that of an 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 amplifier circuit. Therefore:

If we wanted to provide a differential gain of anything other than 1, we would have to adjust the resistances in both upper and lower voltage dividers, necessitating multiple resistor changes and balancing between the two dividers for symmetrical operation. This is not always practical, for obvious reasons.

Another limitation of this amplifier design is the fact that its input impedances are rather low compared to that of some other op-amp configurations, most notably the noninverting (single-ended input) amplifier. Each input voltage source has to drive current through a resistance, which constitutes far less impedance than the bare input of an op-amp alone. The solution to this problem, fortunately, is quite simple. All we need to do is 「buffer」 each input voltage signal through a voltage follower like this:

Now the V1 and V2 input lines are connected straight to the inputs of two voltage-follower op-amps, giving very high impedance.

 

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

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

The Instrumentation Amplifier

As suggested before, it is beneficial to be able to adjust the gain of the amplifier circuit without having 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 is constructed from a buffered differential amplifier stage with three new resistors linking the two buffer circuits together.

Consider all resistors to be 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 be equal to V1.

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

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

That voltage drop causes a current through Rgain, and since the feedback loops of the two input op-amps draw no current,

that same amount of current through Rgain must be going through the two 「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 that all 「R」 resistors are of equal value). Though this looks like a cumbersome way to build a differential amplifier, it has the distinct advantages of possessing extremely high input impedances on the V1 and V2 inputs (because they connect straight into the noninverting inputs of their respective op-amps), and adjustable gain that can be set by a single resistor. Manipulating the above formula a bit, we have a general expression for overall voltage gain in the instrumentation amplifier:

hough it may not be obvious by looking at the schematic, we can change the differential gain of the instrumentation amplifier simply by changing the value of one resistor: Rgain. Yes, we could still change the overall gain by changing the values of some of the other resistors, but this would necessitate balanced resistor value changes for the circuit to remain symmetrical. Please note that the lowest gain possible with the above circuit is obtained with Rgain completely open (infinite resistance), and that gain value is 1.

An instrumentation amplifier is 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 is important to understand the voltage definitions used to describe the amplifier.

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

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

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

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

The output common-mode voltage, Voc, is the average of the two output voltages, and is controlled by the voltage at Vocm.

With a(f) as the frequency-dependant differential gain of the amplifier, then Vod = Vid × a(f).

Basic Circuits

In a fully-differential amplifier, there are two possible feedback paths in the main differential amplifier, one for each side.

This naturally forms two inverting amplifiers, and inverting topologies are easily adapted to fully-differential amplifiers.

Figure 6 shows how to configure a fully-differential amplifier with negative feedback to control the gain and maintain a balanced amplifier.

Symmetry in the two feedback paths is important to have good CMRR performance.

CMRR is directly proportional to the resistor matching error—a 0.1% error results in 60 dB of CMRR.

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 input to the Vocm pin.

With symmetrical feedback, output balance is maintained, and Vout+ and Vout– swing symmetrically around the voltage at the Vocm input.

Generation of differential signals has been cumbersome in the past.

Different means have been used, requiring multiple amplifiers.

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

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

 

 

  

 

A simple IF AGC circuit that features wide dynamic range and excellent linearity can be achieved with two chips:

Tl's TL026C voltage-controlled amplifier IC and Linear Technology's LT1014 (or any other similar basic quad op amp).

 

 

具備50MHZ/-3DB帶寬、20DB壓縮特性的寬帶ALC放大器電路的功能

 

 

這是一種將輸入電平不穩定的信號穩定在必定電平上的電路，用於性能要求高的電路中，在這些信號發生器中，因爲頻率特性不平坦，輸出電平會有波動，

若是加入本電路，則能進行自動控制，使信號保持必定的振幅。此外，爲了下降輸出阻抗，電路加了推輓衝級。

電路工做原理

本電路採用了可由外部電壓控制放大倍數的寬帶放大器IC，從而具備20DB的壓縮特性。輸入電路中，帶有★標記的電阻是爲下降輸入電平而加的，

驅動50歐負載時，由於TL026難以得到較大的輸出振幅，因此在電路中增長了由晶體管組成的推輓緩衝放大器，以減輕TL026的負擔。

TL026的輸出爲差動式，若是負載電阻不相等，頻率特性就會發生變化，因此在引線上接了C2和R4。

引線2.7之間的電位差可對放大倍數進行控制，由於直流漂移，因此用了OP放大器。

二極管D1對輸出進行整流，並與基準電壓進行比較。二極管D2是爲了補償D1的溫度特性而加的。

OP放大器A2起到比較電路的做用，當輸出電平升高時，流過D1的電流就會加大，A2將其積分後輸出負電壓，

並加在A3的反相輸入端，使A2的引線2相對於A1的引線7的電位有增長，從而使A1的放大倍數降低。

元件的選擇

由於整個電路造成ALC環路，因此元件的選用比較容易，可是產生基準電壓的二極管D五、可變電阻VR一、電阻R十二、R13的穩定性則是選用元件時應重點考慮的問題。

爲了使二極管D1和D2的正向電壓相等，應採用熱耦合。普通小信號開關二極管，50MHZ時其整流特性會有所降低，因此，應選用肖特基二極管。

調整和電氣特性

不加帶★標記的電阻，輸入-20DBM，F=1MHZ左右的信號，調整VR1，輸出端得到1VP-P的電壓。

再將輸入電壓放大10DB，驗證輸出有無變化。

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 signal? Thanks.

The PWM is generated based on ADC output.

Hello Frank,

The AGC can be run from the filtered PWM signal.  Keep in mind the limited AGC range, Vref-180mV < Vagc < Vref+180mV. 

This is 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 is 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 have to vary

from approximately 180 mV less than Vref to approximately 180 mV greater than Vref to change the gain from maximum to minimum.

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

∆VAGC = (Vref + 180 mV) – (Vref – 180 mV)

∆VAGC = 360 mV (1)

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

then VAGC should have an ac signal component and a dc component.

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

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

Figures 6 and 7 show two circuits that will perform this operation and are easy to implement.

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

By providing rectification and the required feedback gain, these circuits are 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 two serial cap 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 two 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

採 用 TL026C 的視 頻 光 接 收 機 中 AGC放大 電路設計

TL026C是美 國 TI公 司生產 的一 種具備 自動增 益 控 制 (AutomaticGainControl，AGC)功 能 的差 分 高 頻 放大器。

其增益的改變由AGC管腳電壓控制，相對於 基準 電 壓(REFOUT)對 AGC端輸 入+200mv電壓 ，可 獲得 50dB範 圍的可變增 益 。

TLD26C普遍應用在要 求 寬頻帶 、低 相位偏 差及優 良增益穩 定性 的視 頻和脈 衝 放大 電路 。

TL026C 的 AGC 實現 原 理 TL026C內部 AGC反饋電路使輸出信號具備寬頻帶 、低相位偏 差及優 良的增益 穩定性 。

芯 片增益 的改 變隨 AGC管腳 的控制 電壓 而改變 ，相對 於基準 電壓有 50dB範 圍的可變增益 。

其增益與差分控 制電壓(V — V )的關係如 圖 1所示 。其 中 Vagc是 TL026C的 AGC 管腳 電壓 ，

Vref 是 REFOUT管腳 輸 出 的直 流 電壓 ，是 一 個參 考 電壓 ，其 電壓值 恆 定 ，不 隨芯片 的輸 出電壓 大小改變 。

當 相對 於 Vagc 改變時 ，TL026C芯 片增 益改變 。 由圖 2可看 到 ， Vagc 的值從 一180mv左右 to +180mv左 右時 ，芯片增益 由最大變 到最小 。

即Vagc 當 相對 與基 準 電壓 Vref 增 大時 ，芯 片增 益 減少 ；反 之 ，芯片增益增 大 。

以此 種方法來 實現對輸 出信號增 益 的 自動控 制 ， 進 而使 其輸 出信號保持在一個恆 定的範圍 內。

AGC電路 注意事項

檢波二極 管的導通 電壓決定 AGC電路 的 門限 檢波 電壓 。

硅 管的導通 電壓大約是 0．7V。所以 ，根據輸 出 ，所選二 極管必須使輸 出信號有 足夠的幅度經過 。

因爲 TL026C芯 片 內部 電路 的限 制 ，其最大 信 號輸 出峯一峯值不超過 3V。