When connect sub-circuit together does it change each circuit behavior ?

[heroz]

This sounds stupid question . I'm learning about electronic circuit and I understand analysis of electronic circuit or sub-circuit (for example bjt amplifier , rectifier, differential amplifier , current mirror) but in practice circuit they connected several sub-circuit together
for example op-amp there are several sub-circuit , radio recieveer there are also many sub-circuit.

And I have a question does each circuit change behavior or still the same behavior (for example differential circuit does it change to other function ) when connect to other circuit ?
Because each circuit when connect together there are other component is connected so I think it will change behavior .

 

[KlausST]

Hi,

For sure it has influence:
Since you are not very specific ... let's just talk about signal outputs (source)and signal inputs (load)
Each device datasheet specifies the signal output and input characteristics.
Some devices are designed to have very strong, regulated outputs. Like an OPAMP.
The output impedance usually is below 1Ohm, thus connecting a load of several kOhms won't change the output much
But there are devices with more weak outputs, maybe in the kOhm range, then connecting a low ohmic load will cause to drop the output significantly.

But even if an Opamp output is strong, there still are limits. Like maximum current you may draw.
Let's say 10mA. If you try to draw obove this limit, not only the output will drop, but the internal regulation loop will saturate, and it takes rather long time to recover from this state.

So there will/may be voltage drop, extended timing, overheat, instability .... and so on.

We professionals rely on the datasheet informations and mathematics.

Klaus

 

[heroz]

Hi,

For sure it has influence:
Since you are not very specific ... let's just talk about signal outputs (source)and signal inputs (load)
Each device datasheet specifies the signal output and input characteristics.
Some devices are designed to have very strong, regulated outputs. Like an OPAMP.
The output impedance usually is below 1Ohm, thus connecting a load of several kOhms won't change the output much
But there are devices with more weak outputs, maybe in the kOhm range, then connecting a low ohmic load will cause to drop the output significantly.

But even if an Opamp output is strong, there still are limits. Like maximum current you may draw.
Let's say 10mA. If you try to draw obove this limit, not only the output will drop, but the internal regulation loop will saturate, and it takes rather long time to recover from this state.

So there will/may be voltage drop, extended timing, overheat, instability .... and so on.

We professionals rely on the datasheet informations and mathematics.

Klaus

Thank you. Assume that there are two device when connect together it still work their function right? ( function for example : amplifier , rectifier ) function not change right?

 

[KlausST]

Hi

I can only repeat:

So there will/may be voltage drop, extended timing, overheat, instability .... and so on.

We professionals rely on the datasheet informations and mathematics.

At least "overheat" and "instability" are not considered as "proper working conditions".

Klaus

Added:
See it this way: adding a tyre on a car is possible. Adding the wrong tyres may work or not. Maybe they explode. Although there are videos where cars "work" with exploded tyres ... it's not the the desired operation condition. So car and tyre need to match. You need to read the manuals.

 

[heroz]

Hi

I can only repeat:

At least "overheat" and "instability" are not considered as "proper working conditions".

Klaus

Added:
See it this way: adding a tyre on a car is possible. Adding the wrong tyres may work or not. Maybe they explode. Although there are videos where cars "work" with exploded tyres ... it's not the the desired operation condition. So car and tyre need to match. You need to read the manuals.

Thank you . Sorry to ask you many question . Circuit below :

There are several sub-circuit and I stuck here this sounds stupid in blue section it's differential amplifier right when it work alone but now it connect to other sub-circuit that mean there are several component connect to differential circuit so I think it would change behavior from differential to other function , Is this true or it still work as differential amplifier because when I analyze blue section it still be differential amplifier?

 

[KlausST]

Hi,

There is no simple answer like YES or NO. It depends.

It will have influence: From almost nothing to complete malfunction.
They: you need to treat the signals correctly.

The complete circuit shows an OPAMP.
Blue: differential amplifier. The output is: Q4 collector.
If you treat it as current_souce to drive Q15 base, then everything is fine.
But if you pull this node high with overly current or pull it low with overly current then it will ot work properly.

And for sure if you compare the difference amplifier part with and without connected output: the situation (currents, voltages) will change. Change in a good way.
For sure you may do a simulation with uncennected output, but a circuit with unconnected output is meaningless.

***
Simulation:
There are free simulation tools. Use it, play around, see how the situation changes.

***
If you see the complete OPAMP: Then wihtout external negtive feedback it won´t work properly. The output will saturate. Here you need external circuit to make it work as an OPAMP.
But for sure if you use the wrong feedback network, then you may get it to oscillate.

Klaus

 

[danadakk]

As pointed out its complicated.

In simple terms if we cascade circuits "normally" one thinks of each stage
driving downstream stage as a V source, so that no load effects are created
on the driving stage/interface between stages.

OpAmps good example of very low Zout, over the freq range where they
have G. But as G drops off the Zout rises, looks inductive.

As you can see if we use less fdbk factor (eg. seek higher G) that aggravates the
problem. We get higher Zout at lower frequency. Tradeoffs, always tradeoffs.

Here beta is fdbk factor :

So its complicated. One can use Laplace to predict circuit loading/interaction
at relatively low frequencies. When one gets into RF range the method of analysis
lends itself to using S Parameter analysis (Scattering Parameters).


Regards, Dana.

 

[dick_freebird]

It is a differential input, single ended output (section of a) amplifier. The output had already become single ended before it leaves the block.

Many attribute are not defined by one (arbitrary partitioning) block alone. As one common example your stage gain (DC) is driving gm times net Rout of everything attached. One piece of Rout is the Rin of the gain stage, another is diff stage Rout. Parallel value defines the gain.

 

[BradtheRad]

for example bjt amplifier , rectifier, differential amplifier , current mirror

Among the op amp internals you circled (in addition to the long-tail pair)...
Current mirrors, current sources, amplifiers and their biasing networks. The final (output) stage is a half-bridge.

The purpose of these numerous sub-circuits is to take the differential voltage, and create an output voltage which is: a) a certain value, and b) holds that value regardless what load it sees, or what voltage comes through from the other end. It is done by biasing one or the other output transistors so it turns on in the correct amount.