Electronic Design

Bob's Mailbox

Regarding quad op amps (“What’s All This ‘Free Amplifier’ Stuff, Anyhow?), I thought I’d pass on this tidbit from my early days in the late 1970s. I was working at an industrial controls company on the east coast named Leeds & Northrup Co. (now defunct). (Yeah, I have collaborated with L&N. /rap)

At that time, they were producing a new line of industrial controllers with LED displays, rather than the older analog meter movements, to show process deviation from set-point. A stack of RC4136 “quad 741” op amps was used to drive the LED bargraph display. There were two display modes: single dot and bargraph. (That sounds like a Raytheon part number. /rap)

During burn-in, many of the bargraph displays would go into full-scale oscillation at a rate of about 2 to 3 seconds per cycle. Failure analysis showed that the current required for an RC4136 to drive four LEDs was enough to melt the VSS bond wire inside the package, but not enough to destroy the devICe. The oscillation was the bond wire expanding, detaching, cooling, and re-contacting the bond pads.

(Wild! Was the amplifier in an extremely high-gain circuit? And, was it driving an excessive load? I know a 741 can typICally drive a lot more than it’s rated to, but as you discovered, you can get in trouble doing this. /rap)

These devICes would oscillate in this fashion during days of burn-in and still function just fine in single-dot mode when pulled out for final test! RAY BOWEN

Wow, what a great story! RAP

In looking over the schematIC for the fancy PNP, I could not help but wonder about something (“What’s All This PNP Stuff, Anyhow?). The current sources for sink and source are about 1 mA, while the current that is to be reflected into the output is 0.1 mA to 10 mA. Any mismatch in the sink, and source currents are going to show up in the output. (Yeah, but these currents are going to match well. That’s why I defined those current reflectors. /rap)

I might be missing something. If the base of the PNP is a low impedance point (Isn’t it a negative impedance point? So when current is dumped into it, it comes out the NPN’s collector and comes back out the PNP’s collector. /rap), you have a common base transistor configuration. And in this, the current gain is about 1. There is voltage gain, but the Miller C is removed as it is shunted to ground and does not reflect the driving circuit and cause slow down.

You are a DAC man. Don’t be bamboozled by “paralysis by analysis.” It works! You could even build it in nine minutes.

The Early effect is quite confusing to me. Most textbooks talk about the Early effect only in the CE configuration and do not give finer details. Why are we not observing the Early effect in the CB output characteristICs? (Because it is beta times smaller in the grounded base, so it is just less notICeable. Note: the Early effect in grounded-base is largely invariant of beta. In grounded-emitter, it is largely proportional to the beta. /rap)

Is it because we keep base emitter voltage constant to get constant IE(No. If VBE were kept fixed, the ZOUT would get lousy again. /rap), unlike in CE where we need constant IB? As in CE, IC will increase due to a decrease in recombination at the base and due to an increase in VBE (IC = IS eVBE/VT) but more due to the latter, and in CB, only recombination affects IC. (Not really. You throw that around as if IS were a constant. It is not. See what I said about IS in “What’s All This VBE Stuff, Anyhow? Part 1,” at www.national.com/rap/Story/vbe.html, and in Part 2, June 21, 2007; ED Online 15690. /rap)

If so, then the change in IC that we observe on the output characteristICs shows the effect of base emitter voltage on collector current than that of base-width modulation. Is there a way to find the Early effect in CB configuration experimentally (measure hrb and measure hre; measure ZOUT with fixed base drive, or fixed VBE)? (Why is the output impedance at the collector about the same when IB is held constant or when VBE is held constant? Think about it. /rap)

I read somewhere that since the base is very thin when further reduction happens due to the Early effect, the injection of carriers from base to emitter also decreases. So, this also should decrease the base current. (I don’t think there is anything there to see. When you measure IB, you can’t tell if it is from the injection or from the recombination. /rap) Exactly what things get changed due to the base-width modulation? (Many things. /rap) Please help.

Please, go measure some transistors. Help yourself. RAP

Comments invited! [email protected] —or:
R.A. Pease, 682 Miramar Avenue
San Francisco, Ca 94112-1232

TAGS: Components
Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.