Some basics questions

[Sahara]

i required details of following questions.

1. why we requierd high input resistance? i heard that when we work at very low level than we require high input resistance. can anybody tell us with any application.

2. what is the importance of trigger input coupling in any oscilloscope. like C, HF reject (attenuates >50 kHz), LF reject (attenuates <50 kHz), noise reject (reduces sensitivity). can anybody brief with applicaion.

 

[umery2k75]

No, there isn't any thing like as you have described in (1), I think you are pointing toward the current consumption of devices. Except the BJT devices. Mosftet,JFET and OPAMP have fet input stages, even controllers have fet stages. So they require very low current as an input.All the computational devices that we use like ICs and in discrete component are low powered. So you may see a trend in low power devices that have high input resistance and computational abilities, rather than power devices diodes, bridges, thyristors. We interlink small power(computational devices) to high power devices, like brain operating our body.So you might have seen low powered devices with high input resistance. In high powered devices they have low input resistance.

As far as triggering is concerned, I'm not sure about this whether there are different types of C,HF,LF. When I work with oscilloscope, I haven't come across such thing. About triggering, I used to adjust the vertical knob.Actually it's like when you talk about triggering, there is a signal(Most probably a triangle) by which it compares with the signal coming in the oscilloscope, there by at a point the two waves intersect each other, so you see a sustained wave. So by adjusting the triggering condition you can adjust your wave accrodingly. As far as C,HF,LF is concerned. I think they are built in the oscilloscope probes, because oscilloscope probes comes in wide variety for specific application,people use variety of probes accroding to their application need. Coupling would be done in probe themselves. As often probes have 1X and 10X feature.

 

[edge_tv]

to 1, it depents what you are going to do. Normaly a circuit is used to measure somthing, and that's the beginning of all problems (-:. The main application of an high input resitanc is: not to influenc the cercuit you want to measure, because high input resistanc-> small input current (If you want to masure a Voltage) If you want to measure a current you need a very low resistanc (because you put it in the signal way).So, every time you measuer your measurement produce an error (foundametal problem of Physics). The trick is to make this error so small that it dosen't matter. The moral of the story is, you have to now what you want to do and how your cercuit will chang your signal.

 

[jaywant007]

Now to ques 1

Input Impendence needs to be high or ideally infinite....coz..

when the input source sees the input of your ckt it will see only the input impedance..
this means that if your input impedance is equal to the source impedance then by simple KVL we see that due to the current only half of the Input voltage appears at the input of the circuit...this is known as Input source loading...where the input impedance of the circuit causes the drop in the input....

Now when the input impedance is very high...which means very little current will flow through it...this is like a open circuit simulation....so across the impedance (or simulated open ckt) we get full input voltage...

this is very essential concept in cascading amplifiers...

 

[sujojk]

1) High input resistance/impedance is a desired characteristic for any device. It indicates that the device requires very low current from the previous device (a circuit or part of a circuit) to operate. When input impedance is high, the previous stage can drive infinite (ideal case) number of similar circuits.

Connecting a low input impedance device to a high input impedance will adversely affect the performance of the driving stage. It is because of the paralleling of the two impedances. Since the i/p impedance of the second device is low, the combined impedance will be smaller than the low i/p impedance (resistor paralleling). This will totally change the characteristics of the previous device. Usually this is called loading. The first stage is not capable of driving the succeeding stage.

Again, it depends on your application. If you want to transfer maximum power from source to a device, both the impedances should be equal. (Maximum power transfer theorem). It can be summarized as follows :

If you want maximum voltage (i.e. voltage without any drop) to be transferred to the succeeding stage --->> Second stage's i/p impedance should be high

If you want maximum current to be transferred to the succeeding stage --->> Second stage's i/p impedance should be the lowest possible

If you want maximum power (e.g. cascading of power amplifiers) to be transferred to the preceding stage --->> Second stage's i/p impedance should be equal to the previous stage's output impedance

Note: Here I have neglected the capacitive and inductive part of impedance for the sake of simplicity.



2) Scopes trigger section is used to hold the waveform stationary on CRT (or LCD) display. Triggering is done whenever the signal voltage crosses the trigger voltage set by the "Trigger Level" knob. On a multiple channel CRO, there will be a confusion for which signal should be compared with trigger voltage. That is selected by the "Trigger Source" switch. In a common 2 i/p scope, the usual trigger source options are CH1 (Channel 1), CH2 (Channel 2), EXT (External Signal) and LINE (Trigger from AC Power Line).

The trigger coupling will decide on which component of the trigger signal should be used for triggering. The options are

i) DC - Full signal
ii) AC - AC component of the signal only
iii) HF Reject - DC mode except very high frequency signals are rejected (for better noise rejection)
iv) LF Reject - Low frequency components rejected. Usually used to remove the power supply noise signals.

For our common applications, DC coupling is the most suitable. AC Coupling is best suited when you want to monitor a signal having ac component and a variable dc component. For example, suppose you want to see the ripple waveform of a variable power supply along with the dc waveform. The ripple waveform will be sitting on the top of a constant dc value. At a trigger level you set for a particular power supply voltage, the waveforms will be stationary. But if you change the supply voltage it will go above or below the trigger level which will lead to lose of trigger and scrolling waveforms. (mostly unusable).

But if you are triggering with source coupling and set trigger level almost at the ripple amplitude level, (mV range) the waveforms will be stationary for a very wide range of output voltage of the power supply until its ripple amplitude goes below our set value. So just set the trigger level very close to zero in this particular case.

The HF reject and LF reject coupling are used wherever you want to reject HF and LF noise respectively.


I really hope the above words will clarify your question !!

regards,

SUjO