F
Fred Bartoli
- Jan 1, 1970
- 0
Ok, this is probably a fraction of a degree.
Hunting for a supposed thermal offset in a circuit I set up this little
contraption to get/verify some figures but I fail to see anything...
+40V--+-------+----.
| | |
.-. |/ .-.
adjust | |<---| R3| |2K
clamp | | |> | |
level '-' | '-' 100n
| Q3 | | ||
GND '----+-------||---+---->
| || | 100uV/div
|/ .-. scope input
+25.7V >--| Q2 | |
|> | |1G
| '-'
| |
.-. GND
Rc| |10K
| |or short
'-'
|
|
|/ Q1 = TUT
+5.7V >--|
|> 20K
| ___
+---|___|-.
| R2 |
R1.-. |
2K| | +-||
| | ->||
'-' +-||---< pulse
| |
GND GND
Q1 is the TUT (transistor under test), Q3 is there to clamp the signal
amplitude and avoid thermal distorsion in the scope stages.
With Rc=short Q1 dissipates 5mW or 5.5mW depending on the pulse input state.
Tested Q1 are some TO92 small signal devices, like BC548-9, 2N3904,
2222, ...
Pulse has been varied to check for thermal effects.
Say Tp= 50ms, duty ratio = 20%.
Looking at some moto datasheet for the BC558 we see the transient
thermal resistance to be 0.25*200K/W = 50K/W
500uW*50K/W*2mV/K = 50uV.
I should see it, but nada. Down to the uV level.
Strange...
Hunting for a supposed thermal offset in a circuit I set up this little
contraption to get/verify some figures but I fail to see anything...
+40V--+-------+----.
| | |
.-. |/ .-.
adjust | |<---| R3| |2K
clamp | | |> | |
level '-' | '-' 100n
| Q3 | | ||
GND '----+-------||---+---->
| || | 100uV/div
|/ .-. scope input
+25.7V >--| Q2 | |
|> | |1G
| '-'
| |
.-. GND
Rc| |10K
| |or short
'-'
|
|
|/ Q1 = TUT
+5.7V >--|
|> 20K
| ___
+---|___|-.
| R2 |
R1.-. |
2K| | +-||
| | ->||
'-' +-||---< pulse
| |
GND GND
Q1 is the TUT (transistor under test), Q3 is there to clamp the signal
amplitude and avoid thermal distorsion in the scope stages.
With Rc=short Q1 dissipates 5mW or 5.5mW depending on the pulse input state.
Tested Q1 are some TO92 small signal devices, like BC548-9, 2N3904,
2222, ...
Pulse has been varied to check for thermal effects.
Say Tp= 50ms, duty ratio = 20%.
Looking at some moto datasheet for the BC558 we see the transient
thermal resistance to be 0.25*200K/W = 50K/W
500uW*50K/W*2mV/K = 50uV.
I should see it, but nada. Down to the uV level.
Strange...