some questions about fully differential amplifier

Andrew Wu

Prodigy 470 points

Other Parts Discussed in Thread: TINA-TI

Hi teams,

There are some questions about fully differential amplifier I want to ask.

As the following picture (a),

1、 What is the Rt’s function in the circuit?

2、 If the Rt is a terminal resistance, is it related with the input impedance?

3、 Can you send some useful materials to me about the fully differential amplifier that contain the application of it?

(a)

As the following picture (b),

1、 Is this amplifier application mode same with (a)?

2、 What is the R8’s function in this circuit?

(b)

I’m appreciately waiting for your reply, and thank you!

Regards,

Andrew Wu

over 12 years ago

0 Pete%20Semig over 12 years ago

TI__Mastermind 25815 points

Hello Andrew,

I'm going to move your post to our high-speed amplifiers forum.

Thanks,

Pete

0 Luke LaPointe over 12 years ago in reply to Pete%20Semig

TI__Genius 13335 points

Hello Andrew,

In applications where fully differential amplifiers are used to interface high voltage input signals, Rt and Rs can be used to provide attenuation instead of placing the amplifier in a gain less than 1. This helps keep everything stable and allows low power differential amplifiers to be used in a wider variety of applications.

There is a good series of articles that specifically discusses this point below:

Using fully differential op amps as attenuators:

Part 1 - Differential bipolar input signal - slyt336
Part 2 - Single-ended bipolar input signal - slyt341
Part 3 - Single-ended unipolar input signal - slyt359

Regards,
Luke LaPointe
High Speed Amplifiers

0 Andrew Wu over 12 years ago in reply to Luke LaPointe

TI__Prodigy 470 points

Luke,

Thank you for your reply. But I cannot exactly open the sly336/sly341/sly359. As you say, R8 in picture (b) is the same function with Rt in picture (a)? Is it can constitute a lowpass filter with C5 and C6?

Regards

Andrew Wu

0 Luke LaPointe over 12 years ago in reply to Andrew Wu

TI__Genius 13335 points

Andrew,

The links provided were for a direct download of the PDF. See if these links work instead.

Part 1: http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slyt336&docCategoryId=1&familyId=1453
Part 2: http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slyt341&docCategoryId=1&familyId=1453
Part 3: http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=slyt359&docCategoryId=1&familyId=57

Also, the circuit you posted is in a Multiple Feedback (MFB) bandpass configuration, where R8 and R6/R7 are not acting as attenuators because the configuration is different. R8 does not have an effect on the bandwidth or the gain, but will have an effect on the center frequency of the filter.

In your circuit, the gain of the circuit will be set by: