# I want to use opamp as a linearizer,Can anyone help me. - RF Cafe Forums

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mpatel74

Post subject: I want to use opamp as a linearizer,Can anyone help me. Posted: Wed May 17, 2006 10:14 am

Lieutenant

Joined: Fri Aug 05, 2005 10:05 pm

Posts: 4

Op amp as a linearizer

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fred47

Post subject: Opamp as linearizerPosted: Wed May 17, 2006 7:39 pm

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

There are two ways of linearizing a circuit:

1. Feedback - this is especially difficult to add to an existing circuit at high frequencies, due to the delay around the loop.

2. Cascading a nonlinear circuit with the circuit which needs linearization. In this case, you either need to know the shape of the linearization curve needed, or have some way of measuring the nonlinearity. In either case, the opamp is not the central item.

Do you have any details that can be shared?

Good Luck,

Fred

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IR

Post subject: Posted: Thu May 18, 2006 12:01 am

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello,

Just to add o Fred's second point: In order to linearize a circuit, you will need to add the opposite curve to the circuit you want to linearize (Let's call it DUT), so when you cascade both the outcome is a linear. This is very common in power amplifiers. For knowing the non-linearity you need to measure the AM-AM and AM-PM curves, that means the magnitude and phase behavior of the DUT as function of the input power. Then you have to synthesize the opposite curves of these at the linearizer circuit.

_________________

Best regards,

- IR

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MLR67

Post subject: Posted: Tue Jun 20, 2006 3:30 pm

Lieutenant

Joined: Fri May 12, 2006 11:01 am

Posts: 1

Hello ,

I have just a question I know the problem with feedbak due to delay , nevertheless in the past (for example in Radar) ,feedback was used

.

I know Cherry did a study on this problem.

May you try to explain this problem ?

Cherry, E.M. and Dabke, K.P., "Transient Intermodulation Distortion—Part 2: Soft Nonlinearity," J. Aud. Eng. Soc, 34:1/2, Jan/Feb 1986, pp. 19—35.

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fred47

Post subject: Feedforward linearizationPosted: Tue Jun 20, 2006 6:40 pm

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

There are two kinds of feedback:

1. the normal kind, where a fraction of the signal itself is fed back to the input of a circuit, and

2. the corrector/linearizer kind, where the output itself isn't fed back, just some information about the system performance, to allow an adaptive circuit to adapt. This doesn't require the bandwidth of normal feedback. The classic book on the subject is Adaptive Inverse Control by B. Widrow.

The term "transient intermodulation distortion" came from the audio power amplifier world, where people discovered that listeners could distinguish between two theoretically-identical amplifiers. Those amplifiers had identical frequency responses and steady-state distortion values.

This ability to distinguish amplifiers puzzled many people, and eventually the dynamic behavior of the amplifiers came to be known as the cause. With large amounts of negative feedback, internal signals became large enough to cause clipping - and the beneficial effects of negative feedback were lost.

This is probably going to become more significant in the RF world, as more noise-like signals (such as CDMA and OFDM) are used. These signals have a large peak-to-average ratio, so intermittent clipping is more likely than with more classical signals.

I don't have access to Cherry's article, but "soft nonlinearities" usually refers not to clipping but to active device saturation.

Good Luck!

Fred

Posted  11/12/2012