Tag: CCS performance

CCS: not everything that glitters is gold (Part I)

Introduction

This is the first instalment of a series of blog posts around CCS for valve circuits. Hope you enjoy it as much as I did with the experiments conducted as a result of my interest in CCS-driven circuits.

The depletion cascoded CCS

It’s been long time since I’ve done some circuit analysis and algebra, hopefully I’ve got this right. Seems to get to the expected result, so hey: I’ve done it ok.

The analysis of this circuit starts by using the T-model of the MOSFET. I’ve omitted the parasitic capacitances to simplify the analysis. I leave you the challenge to add them in though. If we look at the typical self-biased depletion FET CCS we can find the output impedance by doing the following formulae crunching:

CCS zout formulae1In summary, the output impedance looking from the source side is:

Zout\approx Rs+\left ( 1+Rs\cdot G_{m} \right )\cdot r_{o} \approx Rs\cdot G_{m}\cdot r_{o} = Rs\cdot\mu

Continue reading “CCS: not everything that glitters is gold (Part I)”

The Noise Inspector

Detecting those little creatures from the bottom

When recently measured the performance of my CCS designs, I found that I wasn’t able to measure below -114dBV (2uV) and therefore limited my ability to measure CCS impedance. I was also keen to measure inductors, transformers and other reactive components to derive 2 or 3 component models for more accurate simulations during the design process. A preamplifier was in order, so I looked at options with some of the ICs I had at hand. A very nice suggestion from Burr Brown/Texas Instruments is shown below. The INA106 is a precision 10x differential amplifier. Not a cheap device, but quality does comes with it unfortunately. When coupled with a pair of OPA37, a very accurate differential preamplifier can be built with extremely low noise and distortion:20140118-184418.jpg

Continue reading “The Noise Inspector”

More on CCS

As I continue with my design of CCS to be used on my next designs as part of the supply filtering stage, I looked at testing the performance of my latest CCS using the following circuit:

CCS test The limitations I have currently is that my waveform generator can do 7Vrms maximum and in low frequency the existing noise level will be the limitation clearly.  As suggested by Gary Pimm, adding a battery operated differential preamp at the point of test will be a great way of raising the low level signal from the sound card interface noise floor. That, will be for a future day. I just want to see how well the CCS performs.

I set the CCS to 30mA and measured attenuation from 50Hz to 30kHz. The results are quite encouraging despite the lack of pre-amplification:

ccs NOISE TEST

The real life CCS is not that great as in the simulation. There is about 10dB difference with the Spice simulation. You can see that I can measure below -130dB attenuation without a pre-amplifier. Still is quite good, more than what we need for.

The CCS is operating to the level of what I need, so test passed 🙂