Comments on: The case for the trans-conducting LNA http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/ Tutorials and Insights in Electronics and Circuit Design Wed, 07 Sep 2011 12:13:21 +0000 hourly 1 By: ashvini vishvakarma http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/comment-page-1/#comment-279 ashvini vishvakarma Mon, 15 Jun 2009 04:55:00 +0000 http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/#comment-279 hi poojan, i love your analog desgn insight posts. 'current mode design' is a bit of a black art for me. and there is only ONE book in entire Amazon on it ! Can you pl write an article on rough rules of thumb to use for (1) when to use current mode and (2) how 2 go about designing in current mode. can you pl also write a post on why the OTA [operational trans-conductance amplifier] failed to 'take over the world' ? what caused its demise ? and is it still used in some neiche ? what is that neiche ? I know about the 4-20 ma current loop of Induatrial, and the teleco current loop for POTS onhook-offhook detection. ashvini, new delhi , india. hi poojan,
i love your analog desgn insight posts.

‘current mode design’ is a bit of a black art for me. and there is only ONE book in entire Amazon on it !

Can you pl write an article on rough rules of thumb to use for (1) when to use current mode and (2) how 2 go about designing in current mode.

can you pl also write a post on why the OTA [operational trans-conductance amplifier] failed to ‘take over the world’ ? what caused its demise ? and is it still used in some neiche ? what is that neiche ?

I know about the 4-20 ma current loop of Induatrial, and the teleco current loop for POTS onhook-offhook detection.

ashvini, new delhi , india.

]]> By: Poojan Wagh http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/comment-page-1/#comment-193 Poojan Wagh Mon, 29 Dec 2008 03:22:43 +0000 http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/#comment-193 @Robert Tso: You're right about the impedance going high outside of the gain-bandwidth of the op-amp. Typically, you'll want to put a capacitor on the inputs of the PMA to maintain a low impedance when the PMA's op-amp (OTA) runs out of steam (and to suck out any LO injection). The addition of such a capacitor is not trivial as it can really screw up stability. I also recall that there's a filter configuration (was it Tow-Thomas?) that includes such a capacitor. I will admit ignorance on many of the details of the BBF, because there was either someone else to do that job (which, I will admit, is the crux of this configuration), or I was always pulled off to other tasks. @Robert Tso:

You’re right about the impedance going high outside of the gain-bandwidth of the op-amp. Typically, you’ll want to put a capacitor on the inputs of the PMA to maintain a low impedance when the PMA’s op-amp (OTA) runs out of steam (and to suck out any LO injection). The addition of such a capacitor is not trivial as it can really screw up stability.

I also recall that there’s a filter configuration (was it Tow-Thomas?) that includes such a capacitor.

I will admit ignorance on many of the details of the BBF, because there was either someone else to do that job (which, I will admit, is the crux of this configuration), or I was always pulled off to other tasks.

]]> By: Robert Tso http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/comment-page-1/#comment-192 Robert Tso Wed, 24 Dec 2008 20:06:17 +0000 http://www.circuitdesign.info/blog/2008/11/the-case-for-the-trans-conducting-lna/#comment-192 If the post mixer amplifier is a low bandwidth amplifier, then it will not do a very good job in presenting a low impedance at the mixer output or at the gm stage output, - so the benefits of improving gm stage linearity would not be realized The likely advantage of the 2nd schematic configuration is that it allows a filter to be placed between the mixer and baseband amp. If the post mixer amplifier is a low bandwidth amplifier, then it will not do a very good job in presenting a low impedance at the mixer output or at the gm stage output, – so the benefits of improving gm stage linearity would not be realized
The likely advantage of the 2nd schematic configuration is that it allows a filter to be placed between the mixer and baseband amp.

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