Stability Analysis and Design of Wideband Howland Current Pumps Using Monolithic Current-Feedback Amplifiers

This paper focuses on the stability analysis of wideband improved Howland currents pumps using monolithic current-feedback operational amplifiers (CFOAs). Based on the application of the Nyquist criterion the possibilities of CFOAs for stable operation in the current pumps at output current i
_L
larger than 1 mA and upper cut-off frequency f
_-3dB
higher than 100 kHz are investigated. Formulas for closed loop gain T = β
⁺
A
_F
^-
of the system with positive and negative feedback at active and complex load are presented. The complex coefficients of the amplifier circuit A
_F
^-
and the positive feedback β
⁺
are determined by using equivalent circuits based on the high frequency CFOA model, taking into account the input impedances, one-pole transfer function, output impedance and parasitic board capacities. Mathematical formulas for the poles and zeros based on the obtained coefficients are derived. Moreover, using these formulas a design procedure of improved Howland currents pumps are proposed. Verification of the defined hypotheses and the proposed formulas is performed by testing of sample electronic circuits using discrete passive components and commercially available CFOA THS3091 with maximum output current ± 250 mA and unity-gain bandwidth ≈ 200 MHz.