Positive and negative feedback loops around A2 combine to create active negative resistance = -R4

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Other Materials from the Main Document

• Article «Digital pot can control gain over a 90 dB span like an electromechanical»
• Figure 1. Grounded wiper makes R2 serve as both input attenuator and output gain set
• Figure 2. R2 has the same function as in Figure 1 with DC bias from R4R5C2 to accommodate bipolar signals. But what about R_W wiper resistance effects?
• Figure 3. Positive and negative feedback loops around A2 combine to create active negative resistance = -R4
• Figure 4. Red curve plots uncompensated R_W (~150 Ω), note the 20 dB loss from both ends of the span. Black curve plots the case where R_W is compensated with negative resistance (R4 = R_W = 150)
• Figure 5. Gain curve using the mechanical pot is identical to Dpot with negative resistance R_W compensation