3904 (again)

[Yzordderrex]

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards,
Bob
N9NEO

 

[Phil Hobbs]

On 2020-03-25 08:56, Yzordderrex wrote:

A client came to me with a battery design that isn't meeting lifetime
spec. The unit is a small transmitter to track people in hospital
type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from
gpio output to the base of common emitter 3904. The collector is
tied to one end of ferrite antenna. Other end is tied to 3v. 3904
is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector
current outside of transistor beta. Battery lifetime is then
dependent on beta

The best solution would be a part that has a narrow range of beta. I
know that gain selected 3904 parts can be purchased, but they are
sorted for minimum gain. I need a part that would be selected for a
maximum gain.

I prefer not to open up the design to artwork changes, but am
thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards, Bob N9NEO

What's the purpose of the transistor? With a 60k base resistor, it
can't be the modulator, surely?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Yzordderrex]

On Wednesday, March 25, 2020 at 9:44:06 AM UTC-4, Phil Hobbs wrote:

On 2020-03-25 08:56, Yzordderrex wrote:
A client came to me with a battery design that isn't meeting lifetime
spec. The unit is a small transmitter to track people in hospital
type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from
gpio output to the base of common emitter 3904. The collector is
tied to one end of ferrite antenna. Other end is tied to 3v. 3904
is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector
current outside of transistor beta. Battery lifetime is then
dependent on beta

The best solution would be a part that has a narrow range of beta. I
know that gain selected 3904 parts can be purchased, but they are
sorted for minimum gain. I need a part that would be selected for a
maximum gain.

I prefer not to open up the design to artwork changes, but am
thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards, Bob N9NEO


What's the purpose of the transistor? With a 60k base resistor, it
can't be the modulator, surely?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

The transistor is a buffer between the crytal oscillator and the ferrite rod antenna. Yes, base drive current is very low. I can't say much more as NDA in place.

 

[John Larkin]

On Wed, 25 Mar 2020 05:56:35 -0700 (PDT), Yzordderrex
<powersupplyguy@netzero.net> wrote:

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards,
Bob
N9NEO

Lots of parts, like BCX70, are sold in fairly tight beta grades.

A bypassed emitter resistor might not control current any better. I'd
have to see the circuit, which I can't, to make recommendations.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[piglet]

On 25/03/2020 2:00 pm, Yzordderrex wrote:

On Wednesday, March 25, 2020 at 9:44:06 AM UTC-4, Phil Hobbs wrote:
On 2020-03-25 08:56, Yzordderrex wrote:
A client came to me with a battery design that isn't meeting lifetime
spec. The unit is a small transmitter to track people in hospital
type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from
gpio output to the base of common emitter 3904. The collector is
tied to one end of ferrite antenna. Other end is tied to 3v. 3904
is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector
current outside of transistor beta. Battery lifetime is then
dependent on beta

The best solution would be a part that has a narrow range of beta. I
know that gain selected 3904 parts can be purchased, but they are
sorted for minimum gain. I need a part that would be selected for a
maximum gain.

I prefer not to open up the design to artwork changes, but am
thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards, Bob N9NEO


What's the purpose of the transistor? With a 60k base resistor, it
can't be the modulator, surely?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

The transistor is a buffer between the crytal oscillator and the ferrite rod antenna. Yes, base drive current is very low. I can't say much more as NDA in place.

Is the 3904 collector load the ferrite rod antenna and is that tuned to
resonance with a capacitor - if so then the load seen is quite high
impedence? 60K means only a few tens microamp base drive so even with
high beta seems hard to get much RF current into the antenna?

piglet

 

[Tim Williams]

So what, is it keyed or something?

If it's class C/D, CW, with a modestly low duty cycle, it can be saturated
and load current will be defined by ferrite antenna resistance instead.
You're in a better place to tell if that's actually the case or not.

Can also swap it to a 2N7002 and a rather smaller gate resistor, but the
higher gate charge would probably end up wasting more power.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Yzordderrex" <powersupplyguy@netzero.net> wrote in message
news:5bf25b81-1873-4886-810b-c75592aa5675@googlegroups.com...

A client came to me with a battery design that isn't meeting lifetime spec.
The unit is a small transmitter to track people in hospital type
environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio
output to the base of common emitter 3904. The collector is tied to one
end of ferrite antenna. Other end is tied to 3v. 3904 is used between
oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector
current outside of transistor beta. Battery lifetime is then dependent on
beta

The best solution would be a part that has a narrow range of beta. I know
that gain selected 3904 parts can be purchased, but they are sorted for
minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a
well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards,
Bob
N9NEO

 

[Rick C]

On Wednesday, March 25, 2020 at 8:56:40 AM UTC-4, Yzordderrex wrote:

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Isn't the current controlled by the inductance of the antenna?
Try adding a resistor in series with the loop and measuring the current. It may not be limited by the current at all. Is there a capacitor anywhere in this loop? If there is no capacitor there will be a DC path that is enabled half the time. So the current to the antenna is a combination of AC and DC with the DC being completely wasteful.

As piglet said, a parallel tuned circuit presents a very high impedance to AC current at resonance. It's still a very low impedance at DC and needs a larger DC blocking cap.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[Rick C]

On Wednesday, March 25, 2020 at 6:54:26 PM UTC-4, Rick C wrote:

On Wednesday, March 25, 2020 at 8:56:40 AM UTC-4, Yzordderrex wrote:
A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Isn't the current controlled by the inductance of the antenna?
Try adding a resistor in series with the loop and measuring the current. It may not be limited by the current at all. Is there a capacitor anywhere in this loop? If there is no capacitor there will be a DC path that is enabled half the time. So the current to the antenna is a combination of AC and DC with the DC being completely wasteful.

As piglet said, a parallel tuned circuit presents a very high impedance to AC current at resonance. It's still a very low impedance at DC and needs a larger DC blocking cap.

Oh yeah, the DC current will be determined by the beta of the transistor if the DC resistance of the coil is low enough.

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

On Wednesday, March 25, 2020 at 8:56:40 AM UTC-4, Yzordderrex wrote:

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards,
Bob
N9NEO

If Vcc is 3V, 60k gives 40uA of base current, or 4-8mA-ish at
the collector.

You can obviously reduce that range with a more complex bias
scheme, but if you don't want to change the artwork, why not
use an hfe-graded transistor as John suggested, and perhaps
a higher-valued (120k?) bias resistor to suit?

It sounds cheesy but it's often done. Garage door transmitters
made in huge volumes use components in selected batches. That
way they can use cheesy bias schemes and still get good results,
for example.

Cheers,
James Arthur

 

[George Herold]

On Wednesday, March 25, 2020 at 8:56:40 AM UTC-4, Yzordderrex wrote:

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Knowing nothing, you could try some similar npn's.
2n4401?
George H.

Regards,
Bob
N9NEO

 

[M Philbrook]

In article <td4n7f56vgac5s4crq3qultqrja7uqf7g8@4ax.com>,
jlarkin@highland_atwork_technology.com says...

On Wed, 25 Mar 2020 05:56:35 -0700 (PDT), Yzordderrex
powersupplyguy@netzero.net> wrote:

A client came to me with a battery design that isn't meeting lifetime spec. The unit is a small transmitter to track people in hospital type environments.

The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904. The collector is tied to one end of ferrite antenna. Other end is tied to 3v. 3904 is used between oscillator and the ferrite antenna.

My thumbnail analysis is that there is no control over the collector current outside of transistor beta. Battery lifetime is then dependent on beta

The best solution would be a part that has a narrow range of beta. I know that gain selected 3904 parts can be purchased, but they are sorted for minimum gain. I need a part that would be selected for a maximum gain.

I prefer not to open up the design to artwork changes, but am thinking a well bypassed emitter resistor may be the way to go.

Any thoughts?

Regards,
Bob
N9NEO

Lots of parts, like BCX70, are sold in fairly tight beta grades.

A bypassed emitter resistor might not control current any better. I'd
have to see the circuit, which I can't, to make recommendations.

Use a CLD (current limited diode), they are low scale devices but work
nicely, that is if you can find a supply for the range you want. They
Also have a threshold point so you need to shop a little.

In any case a device like this can be placed inseries. for that matter
a feed back transistor could be used to clamp the input of the R.F.
output transistor..

 

[whit3rd]

On Wednesday, March 25, 2020 at 5:56:40 AM UTC-7, Yzordderrex wrote:

A client came to me with a battery design that isn't meeting lifetime spec...
The circuit runs off of a 3v battery. There is a 60k resistor from gpio output to the base of common emitter 3904.

First idea: an emitter resistor bypassed with a capacitor.

Second idea: ALD makes MOS devices with adjustable thresholds; combine
a emitter resistor with a grounded-gate NMOS with the drain at 3V, so that
the (negative of the) cutoff voltage of the MOSFET is the sum of Vbe and Ic * Re.
Then Re determines the mean collector current. Emitter-resistor parallel capacitance
is still an option.