# Transistor amplifier -- base voltage?

Discussion in 'Electronic Basics' started by Ken, Jan 24, 2004.

1. ### KenGuest

I am not an electrical engineer and am trying to figure out what
voltage to put on a Motorola mobile speaker squelch lead. My research
has taken me to the [low] limit of my comprehension.

The lead ultimately reaches the base of a transistor amplifier set up
as a "common emitter." The collector is connected to a number of
sub-circuits that apparently effect the squelch function. There is a
table on the schematic that states the following for the subject
amplifier:
unsquelched: base=0.6, collector= 0.4
squelched: base=0.0, collector= 7.5

I take this to mean that a voltage of .6 volts at the base will open
the squelch and a voltage of zero will close the squelch.

My most convenient voltage source is readily adjustable from 0 to 1.2
volts at the base.

(1) Will it harm a transistor amplifier if 1.2 volts is applied to the
base?

(2) What will happen to the collector voltage if the base voltage is
1.2V?

(3) Is it important that I change my source so that the voltage at the
base will never exceed 0.6V?

Ken C

Ken

2. ### Tim DicusGuest

Not normally. Most transistors are rated to take at least 3 volts at the base junction. There are exceptions I understand. Check
It will normally go into a state called "saturation". The collector-emitter junction will pass as much current as it is rated to
carry.
No. Just limit the current to the base with a bias resistor or similar device. My rule of thumb when using a transistor as a switch,
is to use enough base current to send the transistor into saturation, but no more. I have found through trial-and-error that it
takes longer for a transistor junction to come out of saturation the higher the base current was.

Just so you know why sometimes I refer to "trial-and-error", when I got started in electronics, transistors were mentioned in my
textbooks as "future development". That makes me feel old!

Hope that helps,

Tim

3. ### DarkMatterGuest

Telling us that here succinctly dates you as such as well.
Hehehe...

4. ### John LarkinGuest

Likely yes, certainly if your source can supply a lot of currrent. Use
a series resistor, 100 ohms maybe between your source and the base, to
limit the base current.
The transistor will be probably fried, so it could be anything
depending on things. When you apply the 1.2 through a resistor, the
collector voltage should drop to near zero.
Resistor does that for you.

John

5. ### Ian BellGuest

Almost certainly. You need to feed the base from your voltage source via a
resistor. If you voltage source is 1..2 volts then I would suggest a 1000
ohm resistor should do the job.

Ian

Ken C

Ken

7. ### KenGuest

To save bandwidth I did not include more details from the schematic.
The squelch lead passes through a 1K resistor before it arrives at the
base. 0.6 volts is the voltage after the resistor. I figure that the
voltage at the lead that gives 0.6 volts at the base is no more than 5
volts. My convenient source is actually 12VDC, which I figure will
attenuate to no more than 1.2 VDC after the resistor at max base
current. Unfortunately, my graph for base current stops at 0.6 volts
base voltage.

I will start experimenting with a 1 VDC source before the resistor and
gradually increase it to 12VDC, measuring base voltage. When I get to
0.6, that (plus maybe a little more) is where I will stop.

Ken C

Ken

8. ### GarethGuest

The voltage after the base resistor will not double if the voltage
before it doubles. This is because the base current is exponentially
related to the base voltage, i.e. a very large increase in base current
will only increase the base voltage by a small amount. It is therefore
a reasonable approximation to assume that the base voltage will not
exceed 0.6V.

You can now calculate what resistor you need to keep the base current
the same when you use a 12VDC in place of 5VDC.

If in the original circuit 5V was applied to the base via a 1K resistor
the current would have been:

Base current = (5-0.6) / 1000 = 4.4 mA

to get the same base current with a 12V supply you would need:

R = (12-0.6) / 4.4 = 2.59 K ohms.

So if you add a 1.6 K ohm resistor between the 12V and the existing 1K
resistor you should get the same base current as before.

If think the original voltage may have been a bit less than 5V try a  