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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: fast NPN in LT Spice
Date: Wed, 12 Jun 2024 00:35:59 +1000
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On 11/06/2024 7:27 am, legg wrote:
> On Sat, 8 Jun 2024 03:29:11 +1000, Bill Sloman <bill.sloman@ieee.org>
> wrote:
> 
>> On 7/06/2024 11:24 pm, legg wrote:
>>> On Fri, 7 Jun 2024 14:03:16 +1000, Bill Sloman <bill.sloman@ieee.org>
>>> wrote:
>>>
>>>> On 7/06/2024 4:05 am, legg wrote:
>>>>> On Thu, 6 Jun 2024 23:18:12 +1000, Bill Sloman <bill.sloman@ieee.org>
>>>>> wrote:
>>>>>
>>>>>> On 6/06/2024 1:46 pm, legg wrote
<snip>

> 
> Right, I see Q2 normally inverted.
> 
> C2 seems unneccessarily loaded.

If you are talking about the .asc file I posted on the 3rd June, Q2 
isn't "normally inverted" - its mostly on, and it gets turned off for 
the 30nsec in which the stretched pulse is being generated. Inverted 
operation of a bipolar transistor is usually taken to mean running 
current through them in the opposite direction than is seen in normal 
operation.

C2 isn't carrying any current worth worrying about. It stabilises the 
voltage at the base of Q3 against the base current drawn when the output 
pulse is turned on and turned off, -6.7mA when it is turned on an +2mA 
when it is turned off, and holds the voltage excursion down to 3.7mV, 
not that I ever bothered working this out.

If you want to discuss when the circuit does and how it does it, feel 
free, but this wasn't a good start.

-- 
Bill Sloman, Sydney

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