[EdwardM]

;D Absolutely  ;D

Ed

 

[audioguru2]

In a simple cross-coupled two transistor multivibrator the bases of the transistors are driven "below the supply" about half the time.
In a two inverter Cmos oscillator the input of one inverter is driven beyond both supplies on each cycle. ;D

 

[Alun]

Switching regulators can provide voltages way above and below the supply voltage.

 

[Kevin Weddle]

I have seen where a signal can originate at the emitter of a transistor, or at the base. Assuming a base resistor, it seemed possible that the signal applied to the emitter would be larger at the base because of the base current and the resistor. And in fact there is a signal at the base, but it is smalller not larger. Scoff if you will, but it seems logical that the signal could be larger dependent on the base resistor.

It's hard to see, but if you plug in some numbers, here is what happens.

 

[audioguru2]

Kevin,
I don't know what your numbers are supposed to be about.
Aren't you making a Cascode Amplifier?
It doesn't need a collector resistor for its lower transistor.
The voltage gain of the lower transistor is only about 1 becase its high impedance collector is severely loaded down by the very low input impedance of the emitter of the upper transistor. Therefore the circuit performs about the same as a single common emitter transistor at low frequencies.

A cascode amplifier is used at high frequencies because the C-B capacitance of each transistor doesn't cause negative feedback at high frequencies as it does in a common emitter amplifier.

View attachment 37343

 

[Kevin Weddle]

Hi Audioguru, I'm glad you were able to identify the type of cirucit I was describing along with the low impedance observation. I think I used a resistor to increase the impedance. Other than that, here is an acceptable gain circuit. The nice thing about the common base is that the emitter resistor drops most of the voltage creating a large change in current. You know that sometimes you gain a signal only to realize loss somewhere due to the resistances. Aren't we lucky that we can use a resistor here to increase the impedance and at the same time realize a two stage gain amplifier without loss. Amazing.

View attachment 37354

 

[audioguru2]

Sorry Kevin,
Your circuit is a nightmare. Only a single resistor is a positive supply for 2 transistors. If the input voltage becomes high enough then the PNP transistor's base-emitter junction will become reverse-biased beyond its ratings (only 5V to 7V) and have avalanche breakdown.

View attachment 37355

 

[Kevin Weddle]

I just wanted to clarify another point. We are so accustomed to using this principle that we forget what the situation is about. Whenver your dealing with a signal and a DC supply voltage, it's easy to say that the change in voltage over the impedance is the change in current. That is with a DC supply.

In the circuit I posted, If you want to know the collector voltage of the PNP it's the change in current times the resistor. If you want to know the base voltage it's the change in current times the parallel combination of the base resistors.

Now the emitter is different. Since there is an input signal and not a DC supply, the change in current becomes the peak to peak voltage across the resistor divided by the resistor, not the change in voltage over the impedance is the change in current as seen from the emitter of the PNP. The signal originates at the collector of the NPN and divides down. A little different but confusing nevertheless. So it's always best to use the peak to peak voltage across the resistor and not just the impedance to get the change in current.

 

[audioguru2]

Kevin,
What are you talking about?
The signal at the collector of the NPN transistor is small and is simply divided by the voltage divider of its collector resistor and the emitter resistor of the PNP transistor.
The collector of the NPN is a high impedance (just the collector resistor) and the emitter of the PNP is a very low impedance.

Why not just make an ordinary transistor amplifier so that you don't have to throw signal away in a voltage divider? ???
Even better, why not just use an opamp? ???

 

[ante1]

Wow, a dictionary won’t cut it! I need a brain transfusion to follow this one! ???

 

[audioguru2]

Hi Ante,
You wouldn't need a dictionary or a brain transfusion if Kevin made a circuit that can have as much accurate gain or attenuation as he wants, like this one:

View attachment 37363

 

[ante1]

Hi Audioguru,

Thanks, for a while I thought I lost it completely! :-\

 

[Kevin Weddle]

Hi guys. I just wanted to share the problems I was having with theoretical design. I have something interesting that you might like. Suppose the signal was applied to the base of the PNP. And suppose a much smaller signal were applied to the base of the NPN. What I am getting at is that sometimes the impedance of the collector of a transistor can be lowered. I was thinking that maybe the impedance was the change in voltage over the change in current. But of course it is only that way under certain conditions, signal divided to the DC supply.

I hate to bring about the topic of signal mixing as it really complicates things. But what I want to do is show that you can lower the impedance of the collector.

It's not all that funny that I thought the impedance of a circuit was always the change in voltage over the change in current. And what is really isn't funny is that you can design a circuit based on a false assumption with the right resistors. So basically I overcomplicated this circuit originally, but was able to make it work with the right resistors. Never did I know that book examples could be so misleading until I tried to design a circuit.