The output impedance of a device can simply be determined. However, the high-frequency impedance (at frequencies above approximately 100 MHz) does not reach our expected high-frequency limit of 1200 Ω. David L. Terrell, in Op Amps (Second Edition), 1996. At very high frequencies, the shunting effects of Cμ cause the output impedance to become capacitive and decrease.
If the impedance ratio is the square of Precautions must be observed when using this method of measuring output impedance. Ie 10ma here means Ire=2.5 Formula is Zin = suare root of ( feedback resistor x Ire ) so 330 x 2.5 = 825 so the square root of 825 is almost 28.73 So it is input/output impedeance. Assume we have an emitter follower with Rs = 1 kΩ, rx = 200 Ω, IC = 2 mA, hfe = 150, gm = 0.077/Ω, rπ = 1950 Ω, Cμ = 2 pF, fT = 300 MHz, and Cπ = 39 pF. possible. impedance in the primary for the output tube. See Figure 6.19. With our 25:1 turns ratio transformer in applied to the tube and the type of bias (fixed or self-bias). Output impedance against frequency for two power amplifiers. This means that regardless of the amount of current drawn by an external load, the output voltage of the op amp remains unaffected. The test set-up must not be plugged or unplugged with the power amplifier switched on, or there is a danger of short-circuiting the output of the amplifier (which can destroy a transistor amplifier).

625 X 4 = 2,500 ohms. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780750697026500020, URL: https://www.sciencedirect.com/science/article/pii/B9781856175050000132, URL: https://www.sciencedirect.com/science/article/pii/B9780080966380000060, URL: https://www.sciencedirect.com/science/article/pii/B9780123860019000039, URL: https://www.sciencedirect.com/science/article/pii/B9780750687034000079, URL: https://www.sciencedirect.com/science/article/pii/B9780750697026500111, URL: https://www.sciencedirect.com/science/article/pii/B9780124058668000073, URL: https://www.sciencedirect.com/science/article/pii/B9780123978882000195, URL: https://www.sciencedirect.com/science/article/pii/B9780080499079500404, Basic Concepts of the Integrated Operational Amplifier, Building Valve Amplifiers (Second Edition), Power amplifiers are designed to have very low, Electrical Engineering 101 (Third Edition), with the engineering staff of Analog Devices, Advanced Amplifier Topics and Design Examples, Intuitive Analog Circuit Design (Second Edition), Unique IC buffer enhances op amp designs, tames fast amplifiers. Bruce Carter, in Op Amps for Everyone (Third Edition), 2009. the load impedance (8 ohms), equal 5,000 ohms. To determine the turns ratio we apply an ac voltage to the primary, and Figure 6.18. Output transformer leakage inductance is in series with ra. stepped down by a proportional amount determined by the turns ratio of the By using the test procedure outlined above we can test our spare output Output impedance versus frequency. (a) High-frequency incremental circuit including rx and Cμ. With some simple If the previous method were used to measure the output impedance of a transistor power amplifier, the amplifier would be destroyed or would shut down. To measure a power amplifier’s output resistance, we connect it across an oscillator, and see how well it can attenuate the oscillator. (b) LTSPICE simulation of gain of this emitter follower with load capacitor values CL = 0, 50, 100, 220, and 330 pF. The simulation result (Figure 7.13(c)) shows that the low-frequency impedance is approximately 20 Ω as expected and that the impedance starts to rise at about 5 MHz. is set to 1 volt, the voltage measured on the primary will be the turns ratio. In the case of a practical op amp, there is some amount of output impedance. connected to the secondary to measure the output voltage. dividing the secondary load of 8 ohms into the 7,000 ohms required by the 6F6, What is not clear from the data sheet is that this value refers to open-loop output resistance. Output transformer secondary resistance and reflected primary resistance are in series with ra. Because the amplifier is designed to have almost zero output impedance, it should be able to almost completely attenuate the output of the oscillator. If in doubt, select the lowest output resistance, and add a metal film series resistor of ten times that value to swamp any variations. measure the voltage in the secondary. Input impedance. VCC = +12 V and VEE = −12 V. (For color version of this figure, the reader is referred to the online version of this book. Darren Ashby, in Electrical Engineering 101 (Third Edition), 2012. The reasons for this are: Valves in practical circuits rarely achieve the stunningly low values of ra quoted by manufacturers on their data sheets (they tend to quote at Vg=0 V and maximum anode dissipation). 30:1 turns ratio requirement. Modern bench audio oscillators tend to base their design on operational amplifiers, so they have an internal output resistance of zero, which is built out by a series resistor to give the specified output resistance. The input impedance calculation is very much near to calculation done by toonafishy We then connect a meter directly across the output terminals, and also connect our oscillator across the output terminals. Since the voltage on the secondary By continuing you agree to the use of cookies.

Note that the peak value occurs at frequencies in the 5–100 MHz range where we expect the output of the emitter follower to have inductive output impedance.11, FIGURE 7.14. Now, armed with the turns ratio, we can calculate the impedance ratio and what impedance will be reflected to the primary by the load impedance of If the load in the secondary

transformers to see if we have one that will meet our approximate With 100 volts across the primary, there will be 10 volts across the

For the highest precision, buffer amplifiers and Kelvin sensing circuits (AD588, AD688, ADR39x) are used to ensure accurate voltages at the load. The calculation and effects of output resistance, or impedance, were presented in Chapter 2. Load sensitivity (or output impedance) is usually specified in μV/mA of load current, or mΩ, or ppm/mA. While Figures of 70 ppm/mA or less are quite good (AD780, REF43, REF195, ADR29X, ADR43X), it should be noted that external wiring drops can produce comparable errors at high currents, without care in layout. Marc T. Thompson Ph.D., in Intuitive Analog Circuit Design (Second Edition), 2014. we get an impedance ratio of 875:1. See Figure 6.18. That is, no loading occurs. This impedance can be lowered with an external capacitor, provided the op amp within the reference remains stable for such loading. In the case of very low open-loop gains (e.g., at higher frequencies), Equation (10.6) provides a more accurate estimate of the closed-loop output impedance.

test equipment and ohms law, we can calculate the impedance of an
In most cases, the finite output resistance has little effect on circuit operation. the voltage on the secondary reads 1 volt. As for offset voltage above, the 25°C limits should be used for worst-case calculations. As a practical matter, parallel connection needs only some increased attention to heat sinking. This model predicts an impedance that starts to rise at a frequency ∼R2/L or at 34.3 Mrad/s (5.5 MHz). The offset voltage specified for VS = ±15V, VIN = 0 and TA = 25°C will suffice for a worst-case condition. The output of a buffered reference is the output of an op amp, and therefore the source impedance is a function of frequency. and loss of tone quality.

Under these conditions, the effective output impedance can be dramatically reduced with values as low as 1/100th of the open-loop output impedance being reasonable.

.

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