[time-nuts] Switching transistors, current sources, nonidealties and noise

Charles Steinmetz csteinmetz at yandex.com
Mon Jun 20 01:45:24 EDT 2016

Attila wrote:

> Having put the circuit through Spice, I see that the current through
> the tail fluctates violently during the time when the current switches
> from one transistor of the pair to the other. The reason for this seems
> to be that the f_t of the current source transistor is too low to compensate.
> Trying to replace the current source with an RF transitor like BFU520
> that has an f_t of 10GHz helps to dampen these fluctuations by a factor of 2,
> but they are still there.

The transation frequency of the current source transistor is part of the 
cause, but the primary cause is generally the capacitance of the CS 
output node to ground.  Some designers put an inductor in series with 
the output, but I have never found this to be very effective [except in 
poorly-designed simulations] due to the self-capacitance of the 
inductor.  Much better, IME, is to add a cascode device to the current 
source.  (See attached images.)  This has the added benefit of 
increasing the output resistance.  This increase can be very substantial 
(several orders of magnitude) if you use a FET cascode device as shown.

> Why do people use general purpose transistors in these places, even
> though RF transistors definitly improve switching behaviour?
> I dimply remember that someone said/wrote once, that RF transistors have
> a higher noise. But if I look at the datasheet, the quoted noise figure
> for the BFU520 is <1.6dB while the noise figure of the 2N3904 is 2dB best case.

I, for one, have said this, but you are not remembering the whole point. 
  RF transistors are generally considerably noisier AT BASEBAND than GP 
transistors, both because their geometries are inherently noisier and 
because they have *much* higher flicker noise corner frequencies 
(usually 10kHz to some MHz for RF transistors, compared to 10Hz-1kHz for 
GP transistors).  One might think that this would not matter at RF, but 
the flicker noise modulates the bias of the transistor (and sometimes 
other circuit elements), leading to both simple noise modulation as well 
as phase modulation.  RF transistors are not specified for noise at 

Referring to the attached images:

Circuit 1 (files ...01a and ...02a) is an LTspice simulation of a 
cascoded ~3mA current source running into a node that shifts up and down 
by 1v at 10MHz with rise and fall times of 1nS (vaguely simulating the 
emitter node of a BJT differential pair switching at 10MHz).  The green 
trace is the current source output (drain of J1), the red trace is the 
collector of Q1, and the cyan trace is the voltage forced at the CS 
output through its internal 50 ohm resistance.  Only the positive 
transition is shown -- the negative transition is substantially the 
same.  The current increases by ~25% during the 1nS transitions due to 
the output capacitance of the FET.  NOTE:  There will also be stray 
capacitance at the output node, which will make this worse in practice.

Circuit 3 (files ...03a and ...03b) adds an actual differential pair 
with unbalanced 2v p-p drive.  As expected, the simulation shows 
somewhat larger and somewhat longer perturbations at the transitions.

General comments:

Like any simulation, one needs to understand the limitations of both the 
models used and the circuit choices made by the designer.  I cannot 
possibly write enough here to cover the particulars of this simulation 
adequately.  While my experience tells me that my choices are reasonable 
and that the simulation is generally valid, there are still details to 
be mindful of.  You are warned!

I make no claim that these are the best devices to use for a 10MHz TAC, 
although IME they are reasonable choices.

I modeled the ...01a circuit using a BFR90A BJT as the cascode device, 
and the simulation showed that the current spikes were reduced by about 
50%.  However, my experience tells me that this would not hold in 
practice.  The simulation also showed that the DC output resistance fell 
from nearly 300Mohm with the FET to only 1.4Mohm -- which experience 
tells me is in the ballpark of how the real circuits would perform.

Best regards,


-------------- next part --------------
A non-text attachment was scrubbed...
Name: Current_source_01a.gif
Type: image/gif
Size: 10223 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20160620/ce5fd2cb/attachment.gif>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Current_source_01b.gif
Type: image/gif
Size: 10620 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20160620/ce5fd2cb/attachment-0001.gif>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Current_source_03a_w_diff_pair.gif
Type: image/gif
Size: 15172 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20160620/ce5fd2cb/attachment-0002.gif>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Current_source_03b_w_diff_pair.gif
Type: image/gif
Size: 11541 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/time-nuts/attachments/20160620/ce5fd2cb/attachment-0003.gif>

More information about the time-nuts mailing list