But here's an important piece of information: If you are a satellite or digital cable customer, your provider likely already converts the digital signal for you. Basic cable customers could upgrade to digital cable and be covered in the switch. It charges customers a similar amount for modems with its cable service.
Department of Commerce had certified its product as the first digital-analog converter box eligible for the digital television transition coupon program. That box also includes V-Chip parental controls for older televisions that previously did not have them.
Recently, Charter began airing second public service announcements on all networks where it has insertable advertising in an attempt to educate consumers about the change. Some retailers, such as Brown's, are offering free consultations on what customers will need to do to make sure their screen doesn't go black in February To comment, visit suburbanjournals.
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Share this. Retailers are uncertain as to how many consumers will need a converter box. For more information about the digital-to-analog converter box coupon program, visit:. Changing V ref has two important consequences:.
First, it shifts the range of currents that the amplifier is able to measure before saturating. Second, changing V ref also shifts the DC voltage that appears at V div. Remember that the ideal op-amp adjusts its output to keep its input voltages precisely matched.
Therefore, changing the fixed DC voltage at the non-inverting input leads to a change at the inverting input as well. This can be important because the input current source or sink may itself have a limited voltage range over which it can operate. Understanding the input impedance of the op-amp transimpedance amplifier will not only help us manage the stability and bandwidth of the transimpedance amplifier itself, but will also help us design other closely related op-amp circuits like inverting amplifiers and differentiators.
First, observe how the magnitude of the input impedance top graph changes with frequency. Do you notice anything interesting about the shape of the plot? What happens to the input impedance graph? In the simulation, notice also that the input impedance begins to rise decades earlier than the GBW frequency. This is important because transimpedance amplifiers are often used to measure very small currents, and therefore have very large R f , so even a small fraction of it can be large enough to compromise the behavior of the circuit at lower frequencies.
At low frequencies, a voltage increase at the input caused by test current injection is quickly cancelled out by a decrease in the output. High input impedance at high frequencies is the result. The transimpedance amplifier has a particularly interesting result when we find the input impedance Z in algebraically, which helps explain some of the unusual behavior of this circuit.
We hinted at this when we plotted the input impedance on a logarithmic scale in the interactive exercise above. In this case, we want to take the same two circuit equations but use them to eliminate V out instead. When we combine these two to eliminate V out , we find:.
To capture the frequency dependent effects, we can substitute in the Laplace Transfer Function G s of the one-pole op-amp model in place of the DC-only A OL , which is:. As we did in our intuitive input impedance analysis, we can analyze this equation separately for both high and low frequencies.
This matches our earlier work: at high frequencies, the input impedance looks like just the feedback resistor. However, at low frequencies, things get quite interesting. This is large; the equivalent inductances can be quite significant for slow op-amps and large transimpedances.
The inductance is particularly a problem because once we combine an inductance and a capacitance, we have a second-order system and there can be a resonance, where energy repeatedly flows back and forth between the inductance and the capacitance.
The DC input impedance of the transimpedance amplifier is approximately zero. However, when considering higher frequency effects, it would be wrong to assume the input impedance remains zero at higher frequencies because it actually rises drastically. Even worse, when that impedance is inductive in phase, the result can be undesirable instability, resonance, and oscillation.
This problem is most easily demonstrated with a quick simulation where we add small amounts of capacitance C in at the input node:. In the time domain plot, observe the ringing, rather than a clean step response. With even small amounts of capacitance, the simulation shows that this amplifier is only marginally stable. In the real world, it may even be unstable, oscillating uncontrollably. Due to parasitic capacitance , there will always be some amount of capacitance present, even if unintentional.
Just as we addressed stability issues for the op-amp inverting amplifier and op-amp non-inverting amplifier circuits, we can correct for some of the bad behavior caused by input capacitance by adding a compensation network. Relatively speaking, compared to the inverting and non-inverting amplifier compensation examples, compensation is more crucial for the typical transimpedance amplifier because the feedback resistance R f and the input capacitance C in both tend to be larger.
The simplest compensation network is simply to add a feedback capacitor C f in parallel with R f. What does the step response and frequency response look like when this transimpedance amplifier is uncompensated? Look at the ringing in the step response and peaking in the frequency response. Uncompensated, this amplifier is marginally i. The time domain simulation shows substantial ringing, rather than a nice clean step response.
How big should this capacitor be? The easiest way to find the correct value is by simulation. Adjust the parameter sweep for Cf. C to compare multiple capacitor values. Note that even with 0. But when we add just a bit more, we can get the overshoot to go away entirely. Adding compensation to handle instability may be crucial if you want to avoid unintended oscillation in real circuits.
Overcompensation will lower the bandwidth, but will preserve stability and give a clean step response. While compensation does fix the instability, overshoot, and ringing, it does not address the reduced bandwidth of the amplifier. How does the circuit behave with different values for Rf. Clearly, the simulation shows that changing R f alters the DC transimpedance gain, and that lower values of transimpedance do correspond to higher bandwidths.
However, this is a classic gain-bandwidth tradeoff: the magnitude plots all tail off and overlap at higher frequencies! The horizontal section of the magnitude plot is set directly by the value of R f , which varies for each parameter sweep run. These facts explain the shape of the DC Sweep plot: separate traces at lower frequencies, overlapping and declining at higher frequencies.
In fact, this must be the case if the transimpedance amplifier is properly compensated. A larger C f shifts the declining line down. A smaller C f raises the line, until some other bandwidth-limiting element comes into play — in this case the op-amp itself.
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