Maintaining a Vbe Multiplier's bias value
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From: Jim Thompson on 12 Feb 2010 17:21 On Fri, 12 Feb 2010 16:07:52 -0600, krw <krw(a)att.bizzzzzzzzzzz> wrote: >On Fri, 12 Feb 2010 08:24:24 -0800, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Fri, 12 Feb 2010 10:03:11 -0500, Phil Hobbs >><pcdhSpamMeSenseless(a)electrooptical.net> wrote: >> >> >>> >>>Build a man a fire, you keep him warm for a night. Set a man afire, you >>>keep him warm for the rest of his life. ;) >>> >>>Cheers >>> >>>Phil Hobbs >> >> >>Got snow? > >We have about 1" now. They cancelled work for today about 9:30 last >night. Since there was no snow this morning (it started about >10:00AM) I went into work. I was the only one there. My wife's >employer closed about 2:00, with less than 1" on the ground. The >streets are just now starting to get some slush on them. I guess it's >a good thing the locals are scared shitless of snow. I can see the Superstition Mountains from here (they're about 60 miles east of me), no snow even on top of them. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jon Kirwan on 12 Feb 2010 19:59 On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >This is an example of a common emitter voltage amplifier. It might be one of >the easiest stages to design. Normaly, I would start off by knowing what I >need for the stage, for example I need a voltage gain of 20, an input >impedance of 10K, Z out of ?? etc ---- ><snip> I take your point about selecting a BJT for the desired Iq. After that, my next consideration is in setting the collector's average Vc, given an Iq. I want to leave 1V for the dc bias of Ve to stay well above (kT/q)/Iq. And since I want to keep the BJT unsaturated I also preserve 1V for Vce. In your 15V rail case, that leaves me 13V for Vc to wander around in. Half of 13V is 6.5V. Add back in the 2V I'm saving, and that sets Vc at Iq as 8.5V. I set the collector resistor to (Vbat-Vc)/Iq. I get 6.5k for that. What the heck, make it 6.8k. Iq is now 956uA. Oh, well. The emitter resistor is easily set to 1/956uA or slightly more than 1k. Call it 1k. This means Ve will be about 0.956V. Livable. On the biasing, I start by assuming that about Iq/5 should flow in the divider. Say 200uA? (You started out thinking in terms of the thevenin, instead, and a different rule of thumb for it. I just use the 1/5th rule.) Assume Vbe about 0.7V, the base should be at 956mV + 700mV or about 1.66V. Maybe a little less, maybe a little more. Assume less, hope for a little more. So 1.6V/200uA is 8k and (15V-1.6V)/200uA is 67k. Let's go for gusto and pick 68k and 8.2k. ;) Now, I want a gain of 10? I can either bypass the emitter resistor with another R+C in series across it or I can divide up the emitter resistor into two pieces in series and bypass just one of them, leaving the other one active at AC. With the collector resistor of 6.8k, a gain of 10 would suggest an AC resistance of 680 ohms in the emitter. To get that from the 1k DC for the emitter, we put that 680 in series with a 330 and bypass the 330 with a cap sized appropriately for the lowest frequency of interest to be close to a 'dead short.' AC impedance is going to be about the thevenin of the base pair of resistors divided by (1+5/beta) -- the 5 comes from my 1/5th factor I earlier chose. If I keep that very much smaller than the beta, it doesn't affect things much... as you say. The thevenin of 68k and 8.2k is about 7.3k. With a beta of 100, for example, this drops to about 6950 ohms. I'll stop at this point and plug it into LTspice with a 2N2222 model they include (who knows if it is 'good'?) Adding a signal source through a cap to base, I get a gain of 9.6, an average base voltage of 1.58V, average Iq of 921uA, and a center Vc of 8.74V. AC impedance is computed as 6961 Ohms in LTspice. Pretty close, really. Now... to the breadboard for a quick DC check. OnSemi PN2222A just taken from an ammo pack. Collector resistor of 6.800k, emitter resistor 1.008k, base to ground resistor of 8.360k, base to V+ of 69.53k. Measures (rounded): Vbat = 15.1 Vb = 1.59 Ve = 0.95 Vc = 8.73 I haven't hooked up the signal generator, yet. I'll need to move upstairs to do that and get the scope fired up. But that's a quick check of reality. Now, plugging the Vbat back into LTspice and the real values of the resistors I used, I get this from LTspice (rounded.) Vb = 1.59 Ve = 0.94 Vc = 8.8 Which is ... pretty close. (The OnSemi PDF for the part doesn't include a spice model for it and a search on their site only comes up with the MMBT2222, so I'm using the LTspice model for now.) Jon
From: Jon Kirwan on 12 Feb 2010 20:08 On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: > >> >>Jim Thompson wrote in message ... >>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>> >>>>big snip - >>>> >>> >>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>are one (;-0 >> >>I finally did something right? Wow!!! >> > >Actually you found the "art" part needed to rough-in a design. Now go >back and fix the flaws ;-) > > ...Jim Thompson One improvement is to boostrap from emitter to base divider and use a resistor from there to the BJT base, with the signal tying in directly to the base via the cap. Jon
From: Jim Thompson on 12 Feb 2010 20:14 On Fri, 12 Feb 2010 17:08:57 -0800, Jon Kirwan <jonk(a)infinitefactors.org> wrote: >On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: > >>On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: >> >>> >>>Jim Thompson wrote in message ... >>>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>>> >>>>>big snip - >>>>> >>>> >>>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>>are one (;-0 >>> >>>I finally did something right? Wow!!! >>> >> >>Actually you found the "art" part needed to rough-in a design. Now go >>back and fix the flaws ;-) >> >> ...Jim Thompson > >One improvement is to boostrap from emitter to base divider >and use a resistor from there to the BJT base, with the >signal tying in directly to the base via the cap. > >Jon Don't try to get too exotic too fast... you'll get peaking, or worse, oscillation, if you don't know what you're doing. But that's a good thing... try it and learn from it ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jon Kirwan on 12 Feb 2010 20:35
On Fri, 12 Feb 2010 18:14:37 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >On Fri, 12 Feb 2010 17:08:57 -0800, Jon Kirwan ><jonk(a)infinitefactors.org> wrote: > >>On Fri, 12 Feb 2010 14:38:05 -0700, Jim Thompson >><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com> wrote: >> >>>On Fri, 12 Feb 2010 14:25:06 -0700, "bg" <bg(a)nospam.com> wrote: >>> >>>> >>>>Jim Thompson wrote in message ... >>>>>On Fri, 12 Feb 2010 13:43:37 -0700, "bg" <bg(a)nospam.com> wrote: >>>>> >>>>>>big snip - >>>>>> >>>>> >>>>>Son of a gun. Yesterday you couldn't even spell "engineer", now you >>>>>are one (;-0 >>>> >>>>I finally did something right? Wow!!! >>>> >>> >>>Actually you found the "art" part needed to rough-in a design. Now go >>>back and fix the flaws ;-) >>> >>> ...Jim Thompson >> >>One improvement is to boostrap from emitter to base divider >>and use a resistor from there to the BJT base, with the >>signal tying in directly to the base via the cap. >> >>Jon > >Don't try to get too exotic too fast... you'll get peaking, or worse, >oscillation, if you don't know what you're doing. > >But that's a good thing... try it and learn from it ;-) > > ...Jim Thompson Okay. Slow down. So how about a simple (non-Wilson, for now) current mirror in the emitter? It's easy to select a resistor for the other side of it to set the current. Then an R+C leg can be used to set the AC gain! Jon |