What is Input impedance: Definition and 34 Discussions

The input impedance of an electrical network is the measure of the opposition to current (impedance), both static (resistance) and dynamic (reactance), into the load network that is external to the electrical source. The input admittance (1/impedance) is a measure of the load's propensity to draw current. The source network is the portion of the network that transmits power, and the load network is the portion of the network that consumes power.

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  1. brainbaby

    Calculating Input impedance of a CC amplifier

    Hi friends, Here is a CC amplifier The objective is to calculate its input impedance. Two ways are employed. *calculating input impedance using series resistor method (manual method) (refer first two attachment) V1 = 1mV = 0.001 V V2 = 656.4129mV = 0.6564129 V R = 1K Iin = 343.83207µA =...
  2. P

    Engineering Finding out the input impedance in a negative feedback op-amp

    In this case, there is negative feedback. ##V_{out}## is almost equal to ##V_{in}## in this case. But I really can't understand how to even proceed to find out the input impedance in this case, from the equation I know. Can someone help me out in figuring how should I proceed?
  3. putrinh

    How to solve this to calculate the input impedance?

    How did you find PF?: Google search I got a problem, how to calculate this formula from euler to imajiner? or how calculate this case?
  4. E

    Help with Homework Equation: Input Impedance

    Homework Equation: I do not think I am getting the correct answer for this problem when I do it out by hand. Do I plug the necessary values into the equation shown above or do I say that because it is an infinite line that the characteristic impedance is equal to the input impedance? If the...
  5. bobg123

    Engineering Circuit input impedance with ground in an OP Amp circuit

    I've been given the following circuit and have been asked to find the input impedance and the impedance between the input terminal and ground. I've never encountered an operational amplifier configured like this. I know that the voltages at the - and + terminals of the op amp are ideally equal...
  6. C

    Engineering Input Impedance of a MOSFET circuit

    Homework Statement (From Razavi's Design of Analog CMOS Integrated Circuits - Chapter 6) Homework EquationsThe Attempt at a Solution Not too sure about calculating the input impedance of this. Initially I thought that it would be the series combination of C1 and C2, since we are...
  7. G

    Engineering Determine the input impedances of a circuit

    Homework Statement I'm studying a circuit with BJT's and I'm asked to determine the input resistances of the two amplification steps of the circuit. The circuit I'm analyzing is the following one: Homework Equations 3. The Attempt at a Solution [/B] To determine the input resistances I...
  8. Marcus95

    Input impedance of a "Ladder" transmission line

    Homework Statement My electronics&physics lecture notes contain the following side note: ___ "A ladder transmission line comprises an alternating sequence of segments of two different transmission lines both of length $l$ with characteristic impedance $Z1$ and $Z2$. If the line is constructed...
  9. A

    Why does having a high input impedance and low output impeda

    Homework Statement Say you connect Circuit A with low output impedance to circuit B with high input impedance. Why does this cause minimal voltage drop compared to connecting them the other way around (high output to low input). Homework Equations P = V^2/R P = IV The Attempt at a Solution I...
  10. J

    Off center fed dipole input impedance vs height above ground

    I have a question for the community. I am doing a further study and trying to locate any information regarding input impedance of an off center fed dipole at different heights above ground. A simple dipole targets 73.1 ohms for 1 wave length or higher above ground. I am not sure if the same...
  11. maearnie

    Smith Chart: Find Input Impedance for Antenna Problem

    Homework Statement An antenna is modeled by a 40ohm resistor in parallel with a capacitor with Xc = -j25. The antenna is fed by a lossless 50ohm signal line 0.15 wavelengths long. Use a Smith Chart to find the input impedance to the transmission line.Homework Equations The Attempt at a Solution...
  12. T

    Impedance matching a purely resistive load

    Homework Statement A 25-Ω antenna is connected to a 75-Ω lossless transmission line. Reflections back toward the generator can be eliminated by placing a shunt reactance Z at a distance l from the load (Fig. 1). Determine the values of Z and l. Homework Equations...
  13. J

    Input Impedance of off center fed dipoles

    My name is Jackson Richter. I am retired from Rockwell-Collins and worked in the engineering lab for 31 years. Since my retirement my interested have peaked in radio communications. Especially in antenna design. Thanks to anyone here that would give me a little more insight with my...
  14. Planobilly

    How does probe attenuation affect input impedance on a scope

    I assume the 10X setting to be the most useful with audio work. Comments?? My oscilloscope has 1M ohm input impedance when set at 10X as stated above. What is the likely impedance at 1X ? The manual indicates the above chart is based on the probe being set at 10X. As I am dealing with DC...
  15. Hyo X

    Reduce Amplifier Drift when load is near input impedance

    I built a voltage amplifier based on the AD625 instrumentation amplifier. This chip has a 1 G-Ohm input impedance. When the load has a resistance of 100 kOhm, output drift is negligible. When the load has a resistance of 100 MOhm, output drift is significant and not acceptable. Are there any...
  16. AnonymousPT

    Field Effect Transistor - High Input Impedance

    I know FET offers a very high input impedance..but what's the reason behind it? Is it due to the reverse voltage applied at gate with respect to the source which makes the P-N junction reversed biased? If that the case, what happens when there is no voltage applied at gate?
  17. A

    Transmission line input impedance

    Homework Statement (a) A transmission line has a length, ℓ, of 0.4λ. Determine the phase change, βℓ, that occurs down the line. (b) A 50Ω lossless transmission line of length 0.4λ is terminated in a load of (40 + j30) Ω. Determine, using the equation given below, the input impedance to the...
  18. R

    Engineering Understanding Impedance: How to Find Poles and Zeros of a Network

    Homework Statement I have some trouble understanding the worked examples below: Considering the input impedance of the network below: ##z_{in} = R+ \frac{sL/sC}{sL+(1/sC)}## ##z_{in} = R \left[ \frac{s^2+s/(RC)+1/(LC)}{s^2+1/(LC)} \right]## Where ##s=j\omega##. How did they get from the...
  19. D

    Finding input impedance with smith chart

    Homework Statement Find an input impedance of a 50 Ω transmission line of length transmission line of length 3λ/8 terminated with a load of 10 Ω. Homework Equations Zl'=(Zl/Zc) Zin(L)=(Zl-jtan(kL))/(1+jZltan(kL)) The Attempt at a Solution I have uploaded my attempt how far off...
  20. Z

    What is meant by the input impedance of a voltage source?

    I am familiar with the concept of the internal resistance of a voltage source, but what is meant by the input impedance?
  21. S

    What is the input impedance of a FET amplifier circuit?

    Hello all, In one of our labs we determined the input impedance of a FET amplifier circuit via a potentiometer. We were told it was due to "maximum power transfer theorem" (something we have not learned yet). Essentially we attached a potentiometer in series with the input of the circuit and...
  22. G

    Input impedance of a device under large AC signal

    The voltage is large AC signal, not small AC. I never deal with this kind of biasing... I am doing some simulations to extract the impedance of the device. I heard about some ways to measure it, like load pull, S parameter. But I can't not do measurement now... The device is quit...
  23. L

    Input impedance of BJT amplifier

    Homework Statement Find input resistance of the ckt (see attached) Homework Equations The Attempt at a Solution Q2 is diode connected, so I replaced Q2 with VBE resistance (rbe or r∏). So, Rin is rbe1+(β+1)rbe2 But the answer is somewhat different, it's rbe1+ (β+1)...
  24. L

    How Do You Calculate Input Impedance for a Transmission Line with an LC Circuit?

    Hi, I have a couple questions regarding a homework problem and would really appreciate some help. Consider the lossless 50ohm transmission line shown below, which has an LC circuit inserted at some point along the line. If I want to find the input impedance how would I approach this problem...
  25. PainterGuy

    Input Impedance Explained: Example 1-5 Analysis

    Hi, :smile: Please have a see Example 1-5 on the given link: http://img854.imageshack.us/img854/8376/inputimpedance.jpg The example mentions input impedance. What is it in simple words? The book says: Because this input impedance is across the measured terminals , a small current flows...
  26. H

    OP Amp Input Impedance Explained

    Hello Am reading about amplifiers, and the articles i read suggests that for a successfull amplification the op amp should have an input impedance and zero output impedance. I want some explanation about this information, why the input and output impedance are important factors?
  27. R

    Modeling Input Impedance of MESFET Using Series RLC Circuit

    Hi, I'm trying to model the input impedance of a MESFET by using a series RLC circuit. For example I have the following reflection coefficients: .575 angle(-138) at 6 GHz .617 angle(170) at 8 GHz .610 angle(128) at 10 GHz As you can see, as the frequency changes so too does the angle. Does...
  28. W

    Input resistance (Input impedance)

    Homework Statement A digital voltmeter is used to measure the resistance and voltage of a circuit. The resistance and voltage measured is used to calculate current flowing. What errors does the input resistance of the voltmeter introduce into the determination of current?Homework Equations I...
  29. J

    Can anyone tell me what input impedance is?

    Homework Statement Can anyone tell me what input impedance is? The context is "...these two solenoids act as a transformer. An AC voltage Vexp[jwt] is placed across the first and a resistor R across the second. Show that the input impedance of the first solenoid is equivalent to a circuit...
  30. D

    Modeling the Input Impedance of an Amplifier

    I'm trying to model a circuit. Part of this circuit is a guitar amplifier. I don't know what kind of circuitry is inside the guitar amp, it manufactures often list the input impedance in the product specifications. When they say input impedance, do they mean the impedance between the...
  31. Q

    Admittance parameters and input impedance

    http://img420.imageshack.us/img420/9300/img02643tt.jpg hello this is the problem i am working on. i started out by using the admittance parameter equation i1 = y11*v1 + y12*v2 i2 = y21*v1 + y22*v2 so i1 is 20/50 by ohms law. so i1 = 0.4 so by the first equation i get v2 = 32 V. also...
  32. T

    Basic question on input impedance

    Hi, I have a HF transmitter rated with 200mW output power. An antenna is connected to the transmitter via a 50 ohm coax cable. (The antenna is also matched to 50 ohm.) Does this mean that from the transmitters point of veiw, a 50 ohm resistor is connected as load? Can I find the current...
  33. G

    Input Impedance of SIMPLE Circuit

    Can anyone figure out the input impedance of the circuit I have attached? Does anyone know a THOROUGH definition of input impedance? output impedance? Output impedance is equivalent to the thevenin impedance, which mean we turn off all voltage/current sources. But for input impedance...